Washington (CNN) President Donald Trump has begun delivering on his pledge to promote border security and crack down on illegal immigration.

Trump is rolling out executive orders and memos from his agencies, but he will also need Congress and the broader immigration workforce's help to execute many of his ideas.

President Barack Obama actually deported a high number of immigrants as well, but mainly focused on serious criminals as a way to manage strapped resources. Trump's plans are a dramatic expansion of the government's activities, and accordingly are more expensive and logistically challenging.

Here are some of the realities the Trump administration will need to address:

How do you pay for it?

Estimates for the border wall have ranged from $12 billion to $15 billion by Senate Majority Leader Mitch McConnell's best guess to a $21.6 billion option under consideration by the Department of Homeland Security.

And that's just the wall. Trump's orders have also called for hiring 15,000 more border agents and immigration officers, plus boosting detention capacity and hiring more immigration judges.

DHS has limited ability to move money around in its budget, and would almost certainly be unable to find enough resources to fund even most of the new plans.

That leaves Congress: Lawmakers this spring are expected to consider a supplemental funding package that will contain some combination of defense and border security funds.

But GOP leaders have already been put on notice by members of their own party that all this spending had better come with a way to pay for it. And since few Democrats will likely support the bill, the support of Republican budget hawks is critical.

Where will people be detained?

Trump's policies have the potential to massively increase the number of undocumented immigrants held in detention and processed for deportation.

One of Trump's key focuses has been ending what he calls "catch and release," where undocumented immigrants who are apprehended and processed are released into the US on parole pending further court proceedings. It's a way to alleviate the strain on a backlogged immigration court system and detention centers.

But Trump has vowed to end that policy, meaning the government will need a place to put people.

Congress had previously appropriated funding for about 34,000 beds for immigrant detention -- many of which were in facilities operated by private companies across the country. Meanwhile, more than half a million cases are pending in immigration court.

Immigration and Customs Enforcement this week said it has already increased capacity by roughly 1,100 beds since the executive order.

And it's likely private prison companies, which saw their stock go up after Trump's election, will be involved.

How fast will cases be heard?

There are already more than 542,000 cases pending in the backlogged US immigration courts -- a major factor in previous US policies to release individuals pending court dates that could be years in the future.

The new DHS guidance orders the agencies to "surge immigration judges and asylum officers" to try to cut back on some of the backlog, but it's unclear how that will be done. DHS will explore video conferencing systems to make more judges available across the country in any underserved areas as well.

Hiring immigration judges will involve the Justice Department, which runs the US immigration courts. DOJ data for 2014 shows immigration judges making between roughly $125,000 and $170,000 a year, depending on where they are located, so hiring more will add to the total bill.

And of course, hiring depends on the pipeline of qualified applicants.

How do you find 15,000 new immigration agents?

One of the most difficult tasks may be hiring an additional 5,000 Customs and Border Protection agents and 10,000 Immigration and Customs Enforcement officers, as directed by the executive order.

In reality, CBP and ICE have faced difficulties hiring qualified applicants for years.

Homeland Security Secretary John Kelly told the House Homeland Security Committee that he doesn't expect the process of onboarding to be completed "within the next couple of years."

And he pledged to not lower standards to meet the goal.

"We will add to the ranks of the ICE and Border Protection people as fast as we can, but we will not lower standards and we will not lower training," Kelly said. "I'd rather have fewer and make sure that they're high-quality people that are already serving in those organizations, already well-trained, but I will not skimp on the training and the standards."

Hiring border agents has been a persistent problem for the agency, DHS officials told Congress last year. Anti-corruption laws require border agents to pass polygraph tests to combat the possibility that they could be blackmailed or bribed by gangs or criminals.

Even the entrance exam has been a barrier -- 40% of applicants failed to show up or schedule one, officials testified. And it can be difficult to find people who want to work in remote border regions.

Will Mexico cooperate?

US border security policies require the cooperation of Mexico. Beyond any Mexican citizens seeking to cross into the US illegally -- a number that has actually been steadily dropping as Mexico's economy improves -- thousands of migrants from all over the world and Central America make their way through Mexico to try to reach the US each year.

Mexico has in the past worked with the US to cut back on those numbers, and come up with ways to combat drug cartels and gang violence that contributes to people fleeing north.

The Trump administration has had a tense relationship with Mexico from the outset of his campaign, based on his anti-Mexican rhetoric and claims that the nation will pay for the wall, plus talks of possible border taxes.

Trump's policies would also use a US law to send non-Mexican asylum seekers who make it to the US back to Mexico to await their court proceedings in the US -- even working on a video conferencing system to allow them to have their court hearings without returning.

But Mexican officials would have to agree to take these people, and it's not clear how the Trump administration plans to get them to do so.

What will the courts do?

The courts have already been critical of Trump's actions on this front.

A federal district court judge issued a nationwide block on Trump's controversial travel ban, and the challenge to the policy continues on the merits. The Ninth Circuit Court of Appeals upheld the stay as well.

Immigration advocacy groups have vowed to mount numerous legal challenges to Trump's orders on other fronts, as well, on issues including due process, discrimination and federalism. The Supreme Court is currently considering a case on immigrant detention that could impact Trump's policies.

Republicans laid the blueprint, in many ways, by challenging many of Obama's policies in court, often successfully.

Democrats will be looking to do the same to Trump.

CNN's Catherine Shoichet, Scott Bronstein, Curt Devine and Drew Griffin contributed to this report.