This is a nappy that in theory has everything you need all sewn together (in reality you still might need to add a booster). There’s a waterproof outer (normally PUL, sometimes minky which is essentially the same, just fluffy) and then an absorbent core.
Of all the nappy types this one has the most variation between all the different brands. Every single one seems to do their nappies slightly differently.
As you’d expect some are popper-fastened, some velcro. Some brands even offer you a choice of fastening. Some have a stay-dry layer, others will require a separate liner to keep your baby dry. They are normally birth to potty sized, although some brands will also do a newborn version too.
The actual absorbent part varies so much. They can be made from all sorts of different materials, some have more than one layer each made of something different, some have separate boosters. Some have the ability to pull all or part of the absorbent core out to speed up drying time, others do not. This means some require absolutely no preparation once they are washed and dry, whilst others will need re-stuffing.
This really is the one type where unless you’ve done some research into the nappy, you won’t quite know what you are getting. Below are the reviews I’ve done on all in one nappies, these are really worth a watch so you can see the inside of the nappy and make more of an informed decision as to whether it is the right one for you.