They have only a minimal appstore preinstalled, which they use for their own apps. It is the best there is with full background updates etc. Every store could do this if they used modern libraries.
You have Vanadium preinstalled, from which you can install "F-Droid Basic" (the modern client), or Aurorastore, or accrescent, obtainium etc.
Their own solution is sandboxed google play, installed as regular user apps with way more restricted permissions and an opt-in method (only dedicated calls are allowed) in contrast to the extremely privileged microG or even "GAPPS" which can do everything (and in the place of microG having selected things removed, badness enumeration while still using proprietary Google code).
GrapheneOS is basically Android done right, play services etc. work, you can install all Google apps from the playstore, not as system apps (wallet and others are exceptions as they require a Google certified OS).