Hi there,
I believe the previous replies provided enough recommendations to improve yourself with Onshape. I also have three add-ons that I really like and use all the time when I’m modeling with Onshape, and the first one is MKCad, and it’s a library of very popular COTS parts that you can use directly in your Onshape assembly. You don’t have to lose time to import all COTS parts one by one, you can use this library to insert parts in seconds, and there are configurable part options as well. They gave all needed instructions about how to use it in the linked thread.
The other add-on that is used frequently is Some Helpful Featurescripts this one. You can get this easily as a custom feature and use it whenever you want. Especially after pocketing something when I need to fillet each corner I use “Fillet All Edges” from this featurescript. I really recommend it to save some time.
The last one is kind of new and I don’t have so much experience with it but a lot of people liked it.
Announcing KrayonCAD: A Robot Planning Library for Onshape When you’ll be designing a robot and need to see the very basics of each mechanism, you can use this library to create simple assemblies. Especially after kick-off when you’re starting to think about several different designs it’s helpful to know what they will look like and compare each design with KrayonCAD’s you made.
I hope this helps since I think these are some resources that are useful when you start to model with Onshape.