How bodhisattva avoid being enlightened before fulfilling paramita

It's said, someone wishing to become Lord Buddha, doesn't achieve Nirvana until he fulfill his Paramita and goes to last birth (in Theravada tradition). So, while he fulfill his Paramita, - does he consciously do that (avoiding the Nirvana)? (like stopping meditation mid session thinking I should stop now). or - it happens automatically (like, he loses the ability to achieve Nirvana while fulfilling Paramita)? (if, this is the method, that implies that he loses the ability to achieve Nirvana midway, because of a past life decision). or - the midway decision occur randomly and from like 100 beings who fulfil Paramita to achieve state of Lord Buddha, some give up or randomly achieve Nirvana as an Arahant and some give up or randomly achieve Nirvana as a Pacceka Buddha. And, others achieve state of Lord Buddha. So, **what is the process here?** Thanks 🙏.