A beginner question, but my understanding is that BTC (and many others) rely on solving ever increasingly complex algorithmic problems, which acts as a self-gating mechanism on the ever-decreasing cost of CPU power(?)

BTC’s energy use has been in the news a lot recently and there’s discussion about relative transaction cost in other coins. Do they work fundamentally differently, or is their blockchain just so much smaller that they are where BTC was, say, 2 years ago?

Or am I confusing mining with transaction cost?