It's on a voluntary basis. Search for "ancestry world archives project". Familysearch have a similar project for volunteers - with perhaps a better range/variety of records. And UK based there is the FreeCen and FreeBMD project too.
You just do as much or as little as you want - and at present I'm not doing much as I just don't have time.
Both sites will grade projects as easy, medium or hard, and i'd always advise starting with easy projects - that usually means that the source documents are very standard, easy to extract information. Projects in different languages too, you don't need to speak the language to key the information but it obviously helps.
Some od the advanced level records currently on Familysearch are brilliant, Irish church records dating back to 1740, all handwritten. Records may be available on paid sites, but Familysearch do it for free.
I'd really recommend getting involved with one of the projects if you are interested in family history, it exposes you to a wide range of documents and gets your "eye in" with reading old handwriting and knowing what you might expect to find on each sort of document.