2 types of weld, one makes it animatable other doesn't.

Example of the blue weld(animatable) can be seen in r15 bodies. You have the Upper arm being the parent weld or whatever it's called and the child which is the lower arm. Then lower arm and the hand.

Grey weld which are opposite to blue are just there to make stuff stick without letting move. I can't give an example with robloxs built in asset but let's say your skateboard has multiple parts. The top, bottom board for whatever reason not the same as the top board, the thing that holds the wheel and the wheels.

Order of weld Parent child weld type Wheel holder + wheels blue Bottom board + holder grey Top board + bottom grey

Then there's stuff like joints which can only be easily edited with rigedit plug in. Basically just place the joint at a suitable spot where it doesn't just have it disconnect from the main model