Firmware [ Edit ]
Note: If you're new to all this, you may want to skip this page for now, but it's still something that is good to keep in mind (i.e. when you decide to shop for a new hardware).
Most computers these days are using UEFI (or BIOS if you have something older) proprietary firmware, which is extremely bloated and slow monstrocity. To make things much worse, it's paired with Intel Management Engine (ME), which is a backdoor with full access to the all hardware, including the CPU and RAM (which means it also have full access to your running OS). Intel ME is present on all Intel motherboards since ~2006. AMD isn't any better - AMD Platform Security Processor (PSP) is their version of ME and it's present on all their motherboards since Family 15h (Bulldozer, ~2011).
Coreboot is a lightweight, open-source alternative to UEFI/BIOS. Compatible hardware list.
On newer boards, Coreboot still requires binary blobs to function, but on some older boards, it's possible to compile fully blob-free builds. Libreboot is focused on such hardware and, while currently outdated, it's easier to use. List of Libreboot compatible hardware.