This depends on your chosen ROM. Some ROMs, like X-ROM, are based on Samsung's OneUI and include Google services by default. Others, especially AOSP-based GSI ROMs, may require you to flash a separate GApps package. Always check the ROM's XDA thread for the developer's specific instructions.
Open on your PC. Your phone should show up under an "ID:COM" port. galaxy a8 2018 custom rom