A bluetooth amp is similar to any other amplifier used use in an audio system, but includes the ability to wirelessly stream music from a bluetooth source. You don't need a smart phone to bluetooth stream music though. There are MP3 players that will do that. But you will need to load your music on the player.
This is the amp I used, I would not recommend it though. It is very voltage sensitive and if it gets a bit too high (about 13v) it shuts off. I had to build a voltage regulator to feed it. It works now, but occasionally it loses it's pairing and I have to redo it.
https://www.amazon.com/gp/product/B07537PW98/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1Years ago, 1990 to be exact, I drove from San Diego to Memphis using a boombox and a bunch of cassettes for music. I did have an AM/FM radio in my 64 300 but back then music choices across a lot of that trip was limited (C&W or Hispanic) and generally things I didn't listen to. Lot's of D cell batteries were used up on that trip.
