Linux, by definition, begins with the kernel.
https://www.kernel.org/By definition, the kernel is not an OS. An OS includes things like memory management, system calls, process and thread handling.
Can't run the kernel by itself. First you need a way to boot it, like (the already popular) GRUB or LILO.
Then you need some sort of way to output strings and integers on the screen for debugging and a way to handle interrupts and exceptions. Outputting to a serial port can make debugging your new OS easier.
Planning memory usage becomes important at this point. (Assuming you're writing your own Linux distribution...)
Once you're here, some sort of GUI (mouse point and click screen) starts to become important, along with a file system. And depending on what's important to you, your GUI may depend on your file system.
Popular GUIs are GNOME and KDE. There are several others.
Popular file systems in Linux include Ext4 and BtrFS. There are several others. And Linux can access windows file systems such as FAT and NTFS.
And then you need a way to install software and programs other people have already written. There are many of these. Debian uses the APT command, or Synaptic. Fedora uses RPM, YUM, or DNF. And there are several others. Most Linux distributions come with with their own software repositories, or are customized distributions from other distributions that use the same repositories. And there's a movement to try to standardize software distribution among all distributions, as already mentioned, such as SNAP and FlatPacks.
Popular Linux distributions:
Ubuntu
Fedora Project
Arch Linux
Linux Mint
Debian GNU/Linux
Red Hat
OpenSUSE
SUSE
It helps to view the distribution tree occasionally to see where the distribution you're trying to figure out came from.
https://en.wikipedia.org/wiki/List_of_Linux_distributions#/media/File:Linux_Distribution_Timeline_21_10_2021.svgI started out with Red Hat back in the 90's. And I taught Fedora for several years. I'm a 100% Fedora guy now, but I occasionally dabble with other distributions just to see what they're doing. I have a boot disk with Win 7 on it, but I haven't needed to put it back in a computer and boot it in over a year now.