The process is always the same:

You start by reproducing exactly what other folks did. Once you've done that a bunch of times, you unlock 2 skills:

* The ability to handle simple situations that do not require deviating too much from what you've seen in the past

* The ability to learn new techniques simply by reading about them, allowing you to learn much faster

Apply those 2 skills for a couple years (which is not hard at all if you are genuinely attracted to this area) and you are an expert.

What is marvelous is that you don't need to know about those steps, you just follow them naturally when you are passionate about something.

> How do you get started with something like this?

passion is an important part of it, i think almost every obstacle can be eventually overcome if you have the reason to do so

personally if i owned a CarThing, enjoyed using it, and knew it was going to be EOL'd, i would try my best to keep it from becoming e-waste

documenting it makes it even better, since then everyone can share in your passion

> Is it just having a ton of knowledge about hardware / lower-level OSes?

Pretty much, yes. And knowing about common exploit strategies (the crypto engine partial overwrite for example is a classic one).

You can learn things top-down or bottoms-up. I can read & understand most reverse engineering posts like this because I have a strong "bottoms-up" foundation with an EE degree and worked with microcontrollers. But when I read posts by hobbyist mechanical engineers about some 3D printed piston that uses ball bearings I have to approach it in a top-down "recreate what they did and go deep any time I'm lost" manner.

Accumulated trial and error including that which is transferred from others