What skills are need to find ways of take root of Phones?

Question

I am very interested in about take root of Android phones and I'm noob of reverse engineering.

I have some of old Android devices that already unused. Some Android devices provide easier path to root with fastboot mode, But Some of others disabled it by vendor.

What should I find to root non-easier Android device that disabled fastboot mode by vendor? Is it something privilege escalation?

I have no idea what should I find and How to find it.

Thanks for regards.