Using Time Machine with new MBP

Tipitai wrote:

Thank you very much for that really helpful answer. I think it best I get a new external drive to do the back up my new Mac. Do you know if the new Mac would still be able to access the files on my current back up from my old Mac - which was running High Sierra? Really appreciate your assistance.

Yes

If you hold the Option key and click TM icon> Browse Other Backup Disk

Tipitai wrote:

I have a new MBP running Big Sur and used Migration Assistant to copy everything over from my old MBP. Before I did that I did a Time Machine back up of all files from my old MBP onto an external drive. So the files on the external drive and the files on my new MBP are essentially the same. I want to use that same external drive to back up via Time Machine with my new MBP but I don't want/need Time Machine to do yet another back up as it is 514GB. Is there a way I can instruct Time Machine to use all the existing files on the external drive as its base data, and only back up anything from now on which changes? Hope all this makes sense and thanks in advance.

I don't think that is possible....

It will need to index the entire drive because of the new macOS.

if the old mac can now be considered a Back up, you may be better erasing the TM and starting a new, or get a new TM back Drive to use with the new Mac.

It is all a bit confusing—I did not have good luck here and ended up erasing TM and starting over.

If your new Mac inherits your backup history - Apple Support

https://support.apple.com/guide/mac-help/if-your-new-mac-inherits-your-backup-history-mh35732/mac

general rule of thumb:—

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.