If you want to get apps you’ve already downloaded, the only way I know how is by using the Google Files app.

I download all my apps through Obtainium. There may be an easier way than this, but I use all the URLs from the Obtainium export file to go directly to the .apk file. I do this quarterly. I don’t use very many apps, so its not a long process, even if it is manual.

I’ve been saving all the .apk files for apps I find valuable, just in case this ends up impacting future GrapheneOS builds.

Worst case scenario, at least I’d be able to use a saved Pixel for the apps I want, without needing a Google account.