I am not an engineer, but I imagine keeping multiple DC motors running efficiently/in sync together while outside influences change by the second isn’t easy.

It's easy. I'm not a professional engineer, but I'm close enough to know this one.

A typical phone CPU can make adjustments to an output many tens of millions of times a second. It might be "only" thousands for the 10 cent toaster CPU. If it had to model and predict the road ahead somehow, that'd be harder, but just responding to changes as the wheels hit them requires some trig operations at most.

As for the other bit, electric motors are way, way simpler than IC engines, just intrinsically. It's a clever arrangement of magnets, vs a block of metal that has to produce and withstand constant fuel explosions using barely-standardised fuels, and then convert the resulting energy into rotation at the gear ratio of your choice, and do it for years without breaking. With electrics, the magic is all in the battery chemistry.