A lot are just a little electric fence battery in a fancy plastic box. The best one we had got used several times a day, I've started everything from scooters to small diesel trucks with it, and it went strong for at least 3 years before I changed jobs. It was tiny too, you could easily grab it and something else with one hand, then use your other hand to grab the keys, open the door, pop the bonnet release with the other hand, all with minimal effort. Pretty sure it was one of these:
https://www.mrpositive.co.nz/heavy-d...a-jump-starter, but I dunno if we paid that much for it, probably got trade price through BNT or something. I think they come 1500 and 1700 amp