Restore Time Machine backup files after upgrading OS

Finally bit the bullet & upgraded from Mojave to Catalina. As i've seen in other posts there is a problem with accessing all those Time Machine backups prior to upgrading. I can see them on the ext HD, but not accessible when loading TM. The backup dates do show on the right (going back to 2017), but none load on the backup windows on the screen. Only those done post-upgrade. Is there any way to access those files through Time Machine? A ways back (os 10.6?) I was able to access old ones via the cmd or alt keys. I've read that some people do a drag & drop from the files in the Backups.backupdb folder, however I wonder if restoring an application this way will do a complete restore. Please let me now. Thanks!

13.2 iMac (late 2012) running 10.15.7