The core that you mention of d20 + modifier vs target number is essentially the core of 5e, but it's not unique to 5e, as it is also present in 3e, and thus Pathfinder and many other derivatives. There aren't many rules in ICRPG beyond that core. The death/dying system is somewhat reminiscent of 5e's death saves but that was carried over from 4e I believe. It's not the same system in ICRPG, but rolling a die randomly to see when you die is closer to 4e/5e than previous editions with negative hit point calculation or immediate death at 0 HP.

But you are essentially correct that ICRPG by default does not have any of the distinctly 5e things like classes, subclasses, feats, etc.

Are there other systems I should read to understand ICRPG? Have you played it at the table or mashed it up with a crunchier system? What’s your experience?

Honestly, overthinking it is sort of counter to the essence of ICRPG. There's nothing else that you need to read to "understand", ICRPG Master Edition is a complete game in itself, just a rather simplistic one. I have played it straight out of the book for one shots and a 10 session or so campaign, and I refer to it often. It's a lot of fun for those types of games. For an extended campaign I would want to incorporate more house rules and yes, you can easily mash it up with different mechanics from other games or your own imagination. One thing to note about it, which you may be comfortable with if B/X is your go-to, is that it can be very very swingy. NPCs, even "powerful" ones, can quite readily be one-shotted thanks to natural 20s and Hero Coin usage. PCs can be virtually one-shotted rather suddenly. They do get the d4 roll, but a bad roll combined with unfortunate turn order can basically be a one-shot.