In about 18 years in HMO's I've only had one occasion where the rooms were let separately (in that case they were all being let separately from the start). Most of the time you move in and join an existing lease where the leaving tenant is removed and you are added alongside the other tenants.

I think it probably depends on how the initial people moved in. If the estate agent is renting the rooms individually from the beginning it'll be separate agreements. If it's initially rented to a group of friends it's likely a joint AST and then re-assigned over the years as individuals change and the lease is renewed until you have a bunch of strangers jointly and severally liable (not a great idea).