concerning my backup, can I use TM while in Ventura to capture all my apps and app states, documents, etc. .. ?
can that TM backup created in Ventura 13.1 be restored to Monterey after I complete the downgrade ?
Some things yes, some things, no.
Most of the library-based apps change the storage structure with some upgrades. I couldn't tell you which ones might have changed. If they do change, you can't replace them with the ones on the newer OS backup.
If you store everything in the cloud, then it doesn't matter. You would only need to restore other data files.
I thought the application can catch-all in terms of the system state (as a whole), i'm not sure if it works that way or whether i have to restore backed up apps one by one from the backup ..
Time Machine no longer backs up the System Volume at all. To "restore," you install the OS, then Migrate your Data from your Time Machine backup.
You'll need to download the OS you want to use, create a bootable installer from the install app, then startup from that bootable installer. If you have a Mac with a T2 chip, you would need to enable booting from an external drive in the Startup Security Utility.
If it is an Apple Silicon Mac, you just have to create the bootable installer on that Mac. It can be done from another Mac, but a little simpler from the same one.
How to download macOS - Apple Support
Create a bootable installer for macOS - Apple Support
Restore your Mac from a backup - Apple Support
