There are a few steps to achieve what you want to see. I think you may be tripping over a few bit of the GUI prettifying tags to make them readable.
First step - make sure the release you are looking at actually has the instruments in the database. Not every entry in this database is complete down to the instrument levels.
Next up - if you really want to do this manually, you’ll need to have a look at the actual tag names. Which file types are you trying to tag? And which version of tags are you using? Example being that MP3 has a ID3v23 and ID3v24 which is a bit different as to how data is stored as tags. FLAC is different again. This especially affects the instruments.
Highly important is this page in the Picard manual. It shows you how tags are actually put into the files and the slight differences between filetypes
Personally if I am trying to look at which tags are actually in a file, I’ll make use of MP3TAG and the Extended Tags info. Though this is clearly “pretifying” the instruments like Picard does as it splits the performer tag into multiple entries.
When checked with Mediainfo or an EXIF tool you can see that the instruments in a FLAC file are just one long entry called “PERFORMER”.
Richard Wright (hammond organ, minimoog, rhodes piano and synthesizer),David Gilmour (bass guitar, guitar and synthesizer),Roger Waters (effects, guitar, tape and vocoder),Nick Mason (drums and tape),Roger Waters (lead vocals),David Gilmour (other vocals)
Roger Waters (bass guitar);Nick Mason (drums);Nick Mason (percussion);Richard Wright (minimoog);Roger Waters (background vocals);Richard Wright (background vocals);Richard Wright (rhodes piano);Richard Wright (farfisa);Richard Wright (hammond organ);David Gilmour (acoustic guitar);Roger Waters (vocoder);David Gilmour (electric guitar);Richard Wright (synthesizer);Roger Waters (lead vocals);David Gilmour (lead vocals)
Why do you not want Picard to just do this automatically for you? It will save you a lot of headache. It is also not too much of a faff to tweak Picard to only change certain tags.
It all depends on how many files you need to work on.