It was weirdly complex and could basically be summed up as "imagine if all the little side mechanics Sims 4 got from DLCs were actually fleshed out into full fledged mechanics with at least some content to them, it actively simulated the entire neighborhood at once which was also bigger and had more stuff in it, and its difficulty was curved a little more towards actually having to try a little like in earlier games."

It also took forever to load and would actively break without a community patch to regularly fix and clean up invalid background simulation stuff because of compounding errors with said simulation, like background-simulated sims glitching into invalid positions and spamming pathfinding errors - the community patch ran a garbage collection script every in-game day to detect and fix those before they could get out of hand and it worked great. But apart from that it was really good and an iterative improvement over The Sims 2 which had been an iterative improvement over The Sims. It would have been amazing if The Sims 4 had just sort of cleaned it up and kept building on that complexity instead of rebuilding something simpler from the ground up and switching into a minimum-viable-product content churn forever because it's sitting in a niche where it has no real competition at all.