I have had a bad time with PMs and UX designers not actually understanding how the product works _right now_. How can you ask for changes if you don't understan

I have had a bad time with PMs and UX designers not actually understanding how the product works _right now_. How can you ask for changes if you don't understand how it works currently? Like I am saying how the current behavior of the app downright needs a big flowchart to explain and I get asked: "Add X, but keep it working for all existing users" when that means the whole freaking flowcharts needs to be redrawn from scratch. When I suggest to remove some things to make things simpler (because the users don't understand it either) I get denied because it would be too much hassle to communicate the changes.