There is no automatic option to restore backups to an older OS. The reason is that some of your data which utilize databases/Libraries for their data may have changed the layout of the database/library.....meaning some of those apps & services on the older Mac will not be able to access or recognize the data in those databases/libraries.
You will need to setup a new macOS user account during initial setup, then once you are logged into that new user account you can manually select items from the TM backup to restore. I believe you may need to Option-click on the TM icon on the menubar to get an option to browse a TM backup from another system.
The following articles explain the general process, but unfortunately I cannot find an Apple article for browsing a TM backup for another system. I am fairly certain you need to Option-click the TM icon on the menubar though to get such an option.
Restore items backed up with Time Machine on Mac - Apple Support
Recover files using Time Machine and Spotlight on Mac - Apple Support
Remember, items such as the Photo Library, Music Library, Movie Library, and perhaps even the Mail Library may not work when used with an older OS.....I may be missing some, but this gives you a bit of an idea.