How can I save INDD 2022 Documents in a 2021 Version. Do I need an extention?
If you want to share your InDesign 2022 document with someone who has InDesign 2021, save it as an IDML document.
No you can't. If you want a file created in CC2021 to open in CC2022 you don't need to do anything it will open just fine in CC2022. However, for the opposite i.e. opening a CC2022 file in CC2021 you will either have to export the file as IDML or you can also use the online conversion service(it will prompt you if you want to do the conversion when you try to open).
You need to be aware, howerver, that if you used any featers of the new version which are not available ion the older version they will be lost anbd that there isd always the strong possibility of text reflow when moving between versions, and that it may not show until you put the cursor into a story.
As I understand it, the OPs post he wants to provide an InDesign document created in 2022 that it can be opened by someone with InDesign 2021, as the application is not backward compatible it would have to be supplied as an IDML.