I use a phone and a gopro, both of which have somewhat small batteries.
So to power them I use a USB battery which has two USB ports, one for each. Its a little heavy but I'd rather lug that around than have my camera go flat in 90 minutes.
9 Ah battery
2.5 Ah battery
USB connectors are friction fit, which is fine at home on a desk but is not overly secure in the bumpy outside world. So I use quite short USB cables to do the connection, and lay the phone on top of the battery inside a top-tube holder like this:
Some people ride wearing cycling tops, which generally have a couple of big pockets in the small of the back, so you could put a phone and a battery in there on a short lead and ignore it for the ride.
There are also USB power generating hubs, but they're pretty pricey.