Raspberry pi’s use an Arm CPU like a RISC-V chip but different, RISC-V is a little more open source than Arm.

All Raspberry pi’s excluding the pi 5 did not have a power button, really not sure what the power button is for on the pi 5 but they all turn on when power goes to them. So if your power goes out they will turn back on when the power comes back on, but I would strongly suggest getting another boot drive than a SD card. I use the main pi boards, I had magic smoke come from a three I have used numerous pi 4’s, but I currently only have 1 running HAOS and as of this moment I have 3 pi 5’s running things one of those pi 5’s is rather iffy I plan on getting two more pi 5’s in the not to distant future one to become my firewall and one to become my mail server. I have two pi zero 2’s one I used for something and another I will be retiring soon because it only does a half ass job of wireguard.

I have never used anything lower than the original pi zero but if I remember correctly it was a victim of magic smoke as well. From what I can see the pico 2 uses Arm.