There are two main types of orifice for dolls, removable (insert) and fixed (built-in). With removable vaginas you can pull out a sleeve, like a thin fleshlight, after using it. Then you only have to flush this out with warm water and soap, then dry it. You probably also have to wipe some excess lube off the labia of the doll. But the process is pretty quick and easy. You can lube the outside of the insert to make it easier to insert and pull out. I personally recommend removable vaginas, but some say they don’t feel as good, for example because in some models the insert might not fit properly and slide about inside the orifice.
Built-in vaginas are more difficult to clean. Some use soapy water and a sponge on a stick to clean the doll in place, but others recommend carrying it to the toilet or sitting it over a bucket, and flush out the vagina with a douche. Then it needs to be dried, which usually requires some sort of fan or pump (like an aquarium air pump) to ensure air circulation in the orifice. You can also use an absorbing stick stick to dry it, this is even included with some dolls these days. If your doll is silicone, you can also use antibacterial gel to further clean it. However, it is not recommended to use this on TPE, as it can weaken the material.
If you have a built-in vagina, there is also another option, using female condoms. These can be pushed into the vagina and then pulled out when done so that no cleanup is necessary. However some say that it doesn’t feel as good, and sometimes the female condom can break anyway, especially if you reuse them.