For army recruits the recruiting sergeant gave the man a shilling which, once accepted, represented a binding contract. Truth or fable, it is alleged that sometimes the sergeant would go into a boozer and pop a shilling into a man's drink, and he would then be deemed to have accepted it and thus to have joined the army.
The RN made up numbers with the Press but there were also many volunteers in an England where a full belly every night might be a rarity for those at the bottom of the pile. Only those who 'used the sea' were supposed to be taken and generally easily recognised from their dress and gait. Landsmen wer no use anyway. Merchantmen such as Indiamen returning from a two-year round trip might be intercepted in the Channel and the whole crew pressed, and a skeleton crew of men with 'Protections' then put on board to work the ship up to Blackwall or wherever. Pressed men who realised they were really stuck could 'volunteer' at which point they received a bounty and were doubtless then so recorded, whioh would make modern analysis difficult.
I understand the press was normally only needed in wartime when there was both good pay for merchant seamen and a need for many more men for the RN. In peace the steady pay and good food allowances made naval service relatively attractive for seamen, along with security of employement and decent promotion opportunities