While there is no confirmation of how the summoning system works in FEH, below is the general consensus of how the units are generated when you press the summon button:

Units are divided into different pools by star rating, as publicised in the Summoning Rates page.

Randomly choose a pool based on the given probability.

From the pool, randomly choose a unit.

Show the unit’s corresponding color in the summoning circle.

Repeat step one through four 4 times to obtain the full summoning circle.

The most important detail is the 4th step, whereby the summoning circle will show the colors of the units. This gives additional information to the player, which allows them to have some element of control in their summoning sessions. A red orb will always produce a red unit, so on and so forth. This means that a player has some level of control over the units that they will get.

Let’s say for instance that we take a 3 unit standard focus banner with no color sharing, with the probability of getting a focus unit at 3%. With the above method, if a player has no information on the heroes summoned, they will have an equal chance of obtaining each of the focus (1%). However, because they know the color of the available heroes, the player can choose to only summon in that particular color as much as possible, which eliminates the probability of receiving other focus units from the pool.

This does not mean that the odds of getting a particular unit will be the same base rate. For the above example, even if the player eliminates the other focus units by only rolling for one particular color, it does not mean that the probability of the focus unit will remain at 3%, as the number of available units in each color will be different, which results in a different distribution overall.

The general trend is that red has the most units in the summoning pool, green has the least, and blue and colorless are roughly the same. This means that the odds of not getting a red focus are larger than all the other colors, and the inverse can be said for green focuses. This meant that red focus heroes are harder to pull for, while green focuses are easier.

Another mechanic in Fire Emblem Heroes is the pity system, whereby the base rates will be altered according to the amount of consecutive rolls that a player does not receive a 5-star (I will define this as “pity streak”). Every time the pity streak increases by 5, the 5-star rate will increase by 0.5% that is distributed between focus and non-focus, and conversely, 4-star and 3-star rate will drop by the same proportional amount.