It is the same regarding inventory, but different regarding the payments. Both GAM and Admob use Google Ad Exchange inventory, but with GAM you can get better revenue shares w/ Google (compared to Google Admob).
Importing will not cause any problem. To prevent breaking existing projects, if the buildserver finds any Admob component while building the app, it will stop the process.
In other words, you can import apps with Admob projects, but not build/export them. You will have to replace Admob components with Google Ad Manager to be able to generate both APK and AAB.
Finally found a Solution to my Question, I though Google Ad Manager has a smaller Income than Google Admob. Thank you for answering my Questions, now all of the next update changes is clear to me. Have a great day! Thank you for your Hardwork and Efforts in this Platform!