@leberschnitzel "Linux" is a large number of concepts, some common between a wide range of systems, some relatively specific. Fortunately the common stuff is ... more common, and much of the knowledge is highly durable (I cut my teeth about 40 years ago on BSD Unix, the information's served me well over the decades). Even old books can be quite useful, though there is some obsolete data.
One of the best elementary Linux books for several decades has been Mark Sobell's Practical Guide. That's actually a series now, tuned to major distros, and there's an Ubuntu-specific edition. https://www.sobell.com/UB1/index.html
O'Reilly & Associates ("ORA") was long the go-to for technical Unix/Linux books, and I'd recommend both UNIX Power Tools (1992, 2002) which though dated is one of the best introductions to the Unix philosophy and basic shell tools, and Linux in a Nutshell which is a very concise overview of major elements: https://www.oreilly.com/library/view/unix-power-tools/0596003307/ and https://learning.oreilly.com/library/view/-/9780596806088/. Both strongly emphasize terminal / command-line tools.
For general systems-administration guidance, the Unix and Linux System Administration Handbook (a/k/a "Nemeth") remains highly useful, though again, somewhat dated. https://www.colorado.edu/coloradan/2018/12/04/unix-and-linux-system-administration-handbook-5th-edition
I'd also recommend a good book covering your principle shell. These days that's either Bash (the Bourne-Again SHell) or zsh (the "new hawtness"). ORA again has a good bash book: https://www.oreilly.com/library/view/learning-the-bash/0596009658/. They've also got a good zsh guide: https://www.oreilly.com/library/view/learning-shell-scripting/9781783282937/. Other sources might include Sobell, No Starch Press (generally), and Prentice Hall (Sobell's publisher).
There's a whole set of other references, more below.
In general, "learning Linux" is about:
The GUI, for beginners. This is mostly self-explanatory, there are (as with everything else) numerous options, GNOME, KDE, and XFCE are the most popular contenders, with others often based on these, though there are numerous others.
The shell. Covered above (bash/zsh, and others). This is your principle command interface to the system and is both powerful, arcane, and evolving (I've learned and moved on through several shells over my career). Mastering the shell is a key success factor.
The editor. Numerous options, principle are emacs and vim, and I'd strongly recommend you learn at least one of these. Both are available on nearly all systems, including small embedded systems (e.g., modems, routers, Android devices, though often slimmed-down versions (often via Busybox, its own subject...).
Scripting languages. There are several, including not only the shell itself, but old-school options (sed, awk, Perl) and newer arrivals (Ruby, Python, Node.js, Lua, ...), and many others, often obscure and/or specialised. I really only use a few of these myself (sh/bash/sed/awk) and dabble in others (Perl, Python, Ruby), but these are quite powerful.
Packaging. The key distinguishing feature of various Linux systems is the packaging system used, defined by package format, interfaces, and philosophy. Ubuntu uses APT ("a package tool"), the DPKG format, and any of various front-ends. Others include RPM (Red Hat, Suse, and others, often under Yum or DNF), Portage (Gentoo), Slack (Slackware), Nix (NixOS), and more: https://en.wikipedia.org/wiki/Package_manager). Understanding your package manager and its philosophy, or lack thereof, is key to your Linux experience. Best Debian/Ubuntu book here is The Debian System by Martin F. Krafft, https://archive.org/details/debiansystemconc0000kraf. Debian's own documentation is also excellent, see: https://www.debian.org/doc/. (Ubuntu largely follows Debian here, though you might want to check that project's own docs.)
Networking. I won't go into details, specifics vary more on what packages you have installed than on distros per se. But know the basics, as covered in texts above, deferring to your distro's quirks as necessary.
Kernel. I'm going to de-emphasize this relative to others' comments. Yes, the kernel does vary between versions, but ... in general, you will get by well with the stock kernel, and only need to muck with it as new versions come out or you find out specific device or network drivers/modules have issues or are needed. This is increasingly rare if you stick to widely-used hardware and features.
General administration. See the Nemeth and Sobell books for a general overview of topics, but storage, users, permissions, security, and other issues are key here.
Debian (and Ubuntu) have the option to install and manage a huge set of documentation, including but not limited to manual ("man") pages, info documents, HOWTOs, RFCs, and many package-specific guides and manuals. Look for the "-doc" version of packages if you're seeking additional documentation. You can access and search this through a localhost Web interface by installing the dwww and swish2 packages. Point your browser at https://localhost/dwww/ for joy after doing this.
I'll address specific questions on your other toots.