I have done this before, but I never made a video on it because of how time consuming it is.
You can do a few things, you can either completely rebone the entire model and weights, OR you can keep it all and try to find the right bones for the right parts that actually control the model and parenting them to the correct places for VRChat and Unity to be nice to you.
There are bones within the models sometimes that don't actually do anything and there are some that do work as intended, but are not parented to the correct bones or separated completely from them.
Sometimes the models have 3 pairs of bones, sometimes they don't.
It honestly depends on who made it, where they made it and what rig they used.
The process though CAN be easy, but it can also be VERY annoying. It just depends on the model.
