How is process of jailbreaking with locked bootloader done?

You need to OEM unlock the bootloader, which is either as simple as toggling a setting or as hard as getting access to cryptographic keys that are tightly controlled by the OEM (or developing an exploit that circumvents the need for those keys), depending on the phone's OEM. Then you go into fastboot and either flash a custom ROM (if by "jailbreaking" you mean installing a custom OS) or a patched init image (if by "jailbreaking" you mean breaking out of the chroot jail on stock Android)

Can it be automated?

Almost certainly no, as that would require violating several major security boundaries that keep random apps (e.g. malware) from giving themselves root privileges and completely compromising your device. Obviously some devices are accidentally this insecure, and therefore lend themselves to automated jailbreaking, but you shouldn't want that (for obvious reasons)

Although some steps can be automated—e.g. GrapheneOS only requires you to unlock the bootloader, and then takes care of everything else via USB thru their website (and a Chromium-based browser)

1. Unlikely but can happen. Some vendors like Fairphone will fix it for you if you send it in for service.
2. No that is not true, but the most popular Halium based distribution (Ubuntu Touch) has some other peculiarities that make this harder than it should be. They have experimental Snap support now though.
3. Ubuntu Touch is a good start as it supports many devices that are cheap to get. Dual booting is quite hard to do, so not something for beginners.
4. No idea, sorry.
5. Android has a lot of hardware specific battery optimizations. But overall it isn't that bad... the people that complain probably got an old device for testing with a bad battery.
6. I think that is a feature in the latest Android 16.
7. About the same.
8. Device specific and probably not.
9. Afaik no.

Have a look at Droidian - it’s aim is to be as close to Mobian / Debian, but running Halium instead of mainline kernel

Why can you brick android phone by installing custom ROM. But such thing can’t happen on x86?

PCs have a well documented and standard bios (UEFI now), and the process of booting of external media is standard. Plus hardware is extremely custom in ARM land.

Also when people "brick" their phone they usually just loaded a bad rom on it and soft bricked it. Most of the time you're not flashing anything that would actually hard brick it. Delete system32 and now you've "bricked" your PC.