I'm specifically referring to the TV Show, but not sure if the workings of the space suits and helmets are mentioned in the books.
I've been rewatching the TV series again and wondered how the helmets work. There doesn't seem to be any sort of attachments that allow the helmet to connect securely to the space suit/VAC suit (mainly referring to the Belter suits). The helmets just seem to be slipped in over the head, with no pressurised seal around the neck area.
The books themselves are extremely vague on the subject. Typically the act of removing (or opening) a helmet is described simply as "he undid his helmet seals " (Leviathan Wakes) or "he fumbled with the helmet's seals until he managed to pop off the visor" (Leviathan Falls). So there is no real clue if the seals are mechanical, electrical, or something else.
More information is available from the TV series. Belters commonly have a scar around their neck where the helmet joins to the spacesuit, and many of them cover them up with tattoos. The origin of these scars is explained by Dawes in season 1, episode 6: "”The old EVA helmets used to burn like that... The contacts would cook your skin.”
With large electrical currents running through the seals it seems likely, although I don't believe it is stated explicitly, that the seal is made electromagnetically. This is consistent with the book description too.
