How to a install data from a sea gate backup

Did you use Time Machine or some other backup software?

If you used Time Machine, you could have migrated the data during the setup. You can use Migration Assistant to get the data from the backup as long as you didn’t create a user account with the same name as on the old Mac.

https://support.apple.com/en-us/HT204350

If you used some other software, the ask the developer how to restore.

I have used time machine but I also did a complete (I hope) backup about 3 weeks ago.My damaged computer is MacPro 13 early version 2019 which I've been told my data is gone??

So, was your "complete (I hope) backup about 3 weeks ago" a Time Machine backup, or something else? The above-linked article shows you how to use Time Machine to migrate from a Time Machine backup. If you used something else, you'll need to tell us what it was. But, I won't have any idea how to restore from something else.

What did you use to back up to the seagate backup?