Depends on your terms of service and their expiry date. The google term you're looking for is "voluntary separation". Despite what you see in media, nothing in the military bureaucracy is iron-clad. People can and do leave all the time. Usually they keep you for 6 months and then you're off, sometimes without certain benefits depending on circumstances.
The exceptions prove the rule. The situation OP described (military members just ‘quitting’ on short notice and working private sector) aren’t possible for 99% or more of service members.
