How do you make plastic bodies and other stuff for toys and such?

There are a number of ways. I think that commercially they inject plastic into a mold. A DIY project will likely involve kitbashing.
A trailer might be simple enough that you can cut out the plastic sheets that you want, glue them together, and paint them with the desired features.

I think they use molds.

Yes, injection molding is the most common way, but for your purposes, I’d suggest resin casting. Carve whatever you want to make into rubber, mix up some epoxy, pour, wait, pop it out, sand and paint!

Thanks.