digiKam is a DAM (digital asset manager), so it’s for managing your photos: Organising, tagging etc. I believe the Adobe equivalent would be Bridge. It does have an editor, but it’s very basic. It simply shows and manages the files and folders as they are in the filesystem, with (depending on configuration) the database mostly a performance convenience.
darktable is an all-in-one solution, similar to Lightroom. That is, it’s a raw editor with competent DAM/library features. It is database-centric, so once a file has been added it expects that it will stay in the same folder without being moved.
RawTherapee and ART are mainly raw editors. They do have some basic file management, but you may want to use digiKam instead for that.
GIMP is a bitmap editor similar to Photoshop. It has no raw editing capabilities of it’s own (just like PS), so if you try to open a raw file it will send the file to DT or RT to be rendered first. As such, if you shoot raw, it’s only for adding final polish to your edit.
In terms of editing workflow, RawTherapee and ART are going to be the most familiar when coming from Lightroom. It shouldn’t take you long to learn either of them. Darktable, however, is very different from Lightroom and you will basically have to unlearn everything you already know. That means it’s going to take you longer to become proficient, but in return you will gain a level of control over the edit that few other raw editors can match.
RawTherapee and ART
I don’t know of any particular tutorials, but there should be plenty (for RT at least) on YouTube. Since ART is a fork of RT, most basic tutorials will be relevant.
darktable
I recommend that you start by reading the Overview section of the manual. That will ensure you have some important base knowledge. Next watch this beginner tutorial. Details have changed since it was made, so you can mostly ignore where he changes settings, but all important concepts are the same.
The Darktable Landscapes channel has a few videos for users coming from Lightroom: https://www.youtube.com/@DarktableLandscapes
Bruce William’s channel is basically an annotated version of the manual: https://www.youtube.com/@audio2u
Boris Hajdukovic is the darktable wizard: https://www.youtube.com/@s7habo
And all of of these programs have very good manuals, so don’t hesitate to consult them.
No. That doesn’t matter.
There are already several discussions about that here. Short version: Get a CaliBrite Display SL and use DisplayCAL for profiling and calibrating. DisplayCAL tutorial: https://phototacopodcast.com/photographers-guide-to-screen-calibration-with-displaycal/
If you really want to get into the weeds: Articles and tutorials on Color management
That is something you’ll ultimately have to learn through practice, but less is almost always more. A tutorial can teach you how to do something, and even when and what, but rarely how much to do, since it’s so dependent on the photo.