Xoxochippelxo
Apps is a GOS system app. Apps is GOS own "app store" which only updates the apps that it distributes and itself. One of the apps (and its dependencies) distributed via GOS Apps is the play store. This is how you will get and install the google play store.
The apps you install from the play store are updated via the play store itself. And yes, they do receive update and yes you do need to have the play store installed.
If you worry about privacy and security in regards to google play store read this
https://grapheneos.org/usage#sandboxed-google-play
You also have the option to create a secondary user-profile and install and run the play store and the apps you get from it on that profile.