Our docs are being reworked so apologies for that.
A video will be out soon detailing the fragments.
Basically, you add the template parts of your choice inside a fragment. Then, on your post/template, you can insert a Fragment block (and select the fragment of your choice) which will print out the template parts you selected where the position is selected.
You can apply specific visibility conditions to every template part.
This was initially requested here: Hooks for posts, pages, etc - #5 by Takeru