For each new version of SQL Server, the default compatibility level will be set to the version of the Database Engine. However, the previous version's compatibility level is maintained for continued compatibility with existing applications. You can refer to this link: https://video2.skills-academy.com/en-us/sql/database-engine/install-windows/compatibility-certification?view=sql-server-ver15.
I don't know Access particularly well. Do you want to connect to Access data source from sql server? Or connect Access to SQL Server? I found these links for you, maybe you can use them as a reference.
Best regards,
Aniya