There's a great write up on 1d4chan on the AdMech page that does the Mechanicus' knowledge or lack thereof a fair bit of justice.





To paraphrase, even at the height of Mankind's scientific achievements, it's very likely that no one truly understood how their tech worked. Everything advanced beyond a certain point had been designed or at least had their calculations done by AI and other super computers. Once the Men of Iron rebellion happened and the Warning was written against the Silica Animus, the Techpriests were basically forced to use cogitators made from recycled human brains that were incapable of independent learning.





In short, they're trying to maintain a modern nuclear reactor with only access to the ENIAC machine. Understanding the principles is one thing, but when you don't have the means of running the often ludicrously complex calculations to, for example, have a fusion reactor produce more power then it takes to run, knowledge of the principles only gets you so far. It's also why they're so loathe to deviate from the standard template constructs, especially where advanced tech is concerned. Best case scenario, it doesn't work or does so less less efficiently then the STC design. Worst case, it blows up and takes half a continent with it.





And speaking of the events of Mechanicum, let's not forget Mars took an absolute sh*t kicking in the Horus Heresy. Honestly, picture a library full of all human knowledge. Now imagine someone burned that library to the ground, and the AdMech are the poor sods who are sifting through the ashes, trying to piece all the books back together from what half charred cinders remain. Humanity lost tons of knowledge in the Age of Strife, in the Horus Heresy, and with each Forge World lost leading up to the 42nd Millennium. The Quest for Knowledge is very likely life or death for the Imperium, though many wouldn't realize it.







So, in an extremely long winded answer, the Mechanicus does understand how a toaster works. Gellar fields and warp engines... they're a little fuzzier on. All machines are sacred, for all hold a spark of the Machine God. It's just really funny picturing a cult worshiping a vending machine.

