The bootloader functionality is the main thing you really want to know but is hard to find out in most cases. If you can find a machine that accepts custom keys with secure boot you're better off. There are methods that enable secure boot without the ability to add custom keys, but this involves special 3rd party keys signed by Microsoft. It also makes kernel mods a pain if not impossible. The only machines you can fully control are those that can accept custom keys.

There is an excellent guide that describes every aspect of this, including the attack vectors, vulnerabilities, and peripheral uses of the system. It is from the US government here: https://media.defense.gov/2020/Sep/15/2002497594/-1/-1/0/CTR-UEFI-Secure-Boot-Customization-UOO168873-20.PDF

The only other reference I have found with additional information is from a Gentoo guide that describes how to boot into the UEFI system and make changes directly. This may be an option if you can't alter secure boot.

https://wiki.gentoo.org/wiki/User:Sakaki/Sakaki%27s_EFI_Install_Guide/Configuring_Secure_Boot

Again, this only really applies to modern hardware with secure boot, and only in instances where you may need to run custom kernels or modules other than those that come presigned by distro packagers using Microsoft's 3rd party key.