I have just installed the app from the old mac to the new M1 MacBook Air and when I try to open it it says To open Photoshop 2020 I need to install rosetta in order to run it. What is it and is this the right way to do it?Or is there another version of Photoshop that is compatible?
Here is Adobe's Help page as to when to open Photoshop in native mode or Rosetta. Some PS functions are not yet compatible with native mode. If you use those, you will need to open Photoshop in Rosetta.
Photoshop 2020 is written for Intel processors, not Apple Silicon processors. Rosetta is translation software that Apple provides so that old Intel programs still run on an Apple Silicon Mac. If Rosetta wasn’t there, then none of the Intel-based software you migrated would work at all on an M1 Mac. However, running through translation is not as fast as if a program is written specifically for Apple Silicon.
The latest version of Photoshop is written specifically for Apple Silicon. It runs much faster on your M1 than the old Intel-based Photoshop 2020. Use the Creative Cloud desktop app to upgrade to it.
Outside of Photoshop, you will get that Rosetta message any time you start an application (from any company) that has not been updated for M1. If you don’t want to see that message and want to use the fastest version of any application for your M1 Mac, see if the latest version of that software has been updated for M1, and if so, update to that.