When an option is issued, it has an associated vesting schedule. This can either be that the options all vest at the same time, or in tranches.
When a vesting date has passed (whether tranche or whole vesting), the company is contacted to confirm that the vesting should happen.
This is done digitally via the platform, can be in whole or in part for the relevant options, and the recipient is informed accordingly.
In addition, if the option agreement allows it, the company has the ability to accelerate the vesting if it so wishes (or indeed reverse vesting that has already occurred).
This means, that in the case of someone leaving the company, their options can be set to allow any specific number to be vested as determined between the company and the individual.