Windows, iOS, MacOS and default ROMs for phones made by big corporations have many backdoors and they can all be shut off with one click. Use of Open Source is a must since u know what u are running, but even with that u cant really know what is happening on the hardware level, so always look as if there will be 0 electronical devices working. It is recommended to never connect these devices to the internet.

Any old tech, the older the better (pre 2010). Phones are recommended because they are low on power and more compact, old computers and routers should be bought just incase. SBCs (Orange PI, Banana PI) and PCBs (Older arduinos, esp32s) can be bought.

For computers use any linux distribution (avoid ubuntu and similiar) and for phones PostMarketOS, LineageOS or any Open Source ROM.

No AI guide has list of usefull apps:

Solar foldable usb chargers and bike generators are good options. Old phones can have bad batteries which are risky so making a non-battery powered mod is a good thing.

Faraday bag is a good start make sure to have plastic/metal/rubber boxes, anti-static bags and tin/aluminium foil.