First, let me be perfectly honest, I am not the Pool Layout Expert, not by a long shot.
Which drive to replace first, it doesn’t matter as far as I am aware. You have a RAIDZ3 so you are protected from 3 drive failures so makes no difference. I personally think I would replace the failing spare first, but that is purely a personal choice. And let it all resilver. Then I would run a Scrub, you should not need to, however it is a personal thing. Then replace the drive in the pool.
As for instructions, I entered “How do I remove a spare drive from core?” and it provided the answer.
I will say, the AI help was terrible in the beginning and it is getting much better over time. The information is likely in there but the wording of a question may need to be slightly different.
The pool layout, as I said, I’m not expert. At least your IT person did a RAIDZ3 for such a large amount of drives in one VDEV, in your case one VDEV=your pool. For now, get your system back to 100%. Then you can read what the AI will tell you if you ask it this “what is the best pool layout for 24 drives?” and it will tell you some stuff and provide a link. Here is a link: ZFS Pool Layout White Paper