The new UI is completely different and none of the previous ways of customizing will work.
The new ones are pretty much undocumented, although there is something about this in the Docs, and there is the generic Angular knowledge that will surely be helpful.
Your quickest way to get back to a safe point is to turn off the new UI on the specific modules you’ve customized - this should make v7 stuff work again. Search these forums for how-tos, it’s been discussed often. Here’s one example: