Ok, let's take the easy ones first. All you need to do to move files and folders using the external is to copy them to the external. If all of it is in his home folder, you can just copy the "Documents" folder there to the external and that should do it. Then, when you move the external to the new Mac, copy all of what is in "Documents" in the external to the Documents folder in his user area there.
Pictures and videos depend on how he has them organized. If they are in Photos (or iPhoto, I'm not sure which version of macOS made that change) on the old system, then there should be a library file holding those images. You can launch Photos/iPhoto on the old system, the look in the "preferences" for where the library is located. If it was already in "Documents," then you already have it on the external with the earlier move. If it is somewhere else, copy that location folder to the external and then back to the same location once you attach to the new. You'll have to point Photos to that location in Photos Settings, and if it was iPhoto-created, then it will convert to Photos format, or should. You may have to import to Photos from the iPhoto library. I'm not really sure, it's been a while and the conversion was done on my system when I did the upgrade that brought on Photos.
If the pictures and videos are kept separate in folders in his Documents folder, you've already copied them. If they are in some other location, again, find them and copy that location to the external.
OK, now for the applications. I don't know any of them. But look at their preferences to see if they have a preferred location for storing stuff, and if so, again, if it's in his Documents folder, you already have it copied. If not, then find it and copy that location to the external just as with the pictures.
Now, for each application itself, I would recommend looking at the developer site(s) to see if there is a version for Ventura and his new Mac (I'm presuming it's an Mx Mac, not an Intel, but you didn't say.) If they have a version to suit, I would recommend downloading it and installing it fresh. Then try to use the Settings to point to where you put the old data to see if the new install can read that data. If it's a game, the history will probably be lost in this process. Migration Assistant would probably keep that history for him, but doing it manually generally sets it all back to zero.
The nice thing about Migration Assistant is that it does all of the housekeeping for you and can even preserve history for you because it reads the various configuration files and preserves them if it can.
For the mouse, get the latest Logitech driver for it for the new Mac. One can hope they have updated it for Ventura/Mx, at least.
Hope that helps some. Maybe somebody can chime in here if I've missed anything. Not moving Mail and Messages makes it a WHOLE lot simpler!
