

You have the bootloader, the initramfs, the boot procedure, systemd, PID 1, you can check the logs, you have all those little things that make the system work
Windows has pretty much the same concepts. These two books talk about all that in deep:
https://www.microsoftpressstore.com/store/windows-internals-part-2-9780135462409
Also check out this one: https://leanpub.com/perfbook
The Umbrella Academy: in the first couple of series like nothing happens and everyone is very sad.