I posted this before looking for topics, but it kinda ended up being online support for general bicycle troubles.
Which is a good thing! Ask your questions about bicycles that are currently on your mind and we all try to help.
Pre-Emptively calling in @dallasw and @Kissmydadonthelips for their knowledge.
how do those batteries i see couriers attach to their frames work and is it difficult/expensive to install for ones self?
Well all E-Bikes need a battery and you have to attach it somewhere and then make cables run to the motor.
Which is where the confusing part begins, as there's many types. Mostly depending on where you are, this runs from a motor you can engage with your thumb to some type of pedal assist.
The first one is pretty self explanatory, push the button and things go fast.
The second one get's a bit harder as there's very real differences as to whether a motor picks up your speed and then just goes woosh as is often found in front- or rear wheel mounted motors or whether it does some math bullshit to basically do your pedaling but multiplied by factor X.
Probably not too expensive and the difficulty depends on whether you know your way around a wrench, I'd still advise against it though. Your usual bike components aren't made to withstand the torque of an electric motor constantly is the problem.