What clarification questions can be asked to an employee who will have to support projects instead?

Question

1C-Bitrix

I would like to know from those who have had to deal with such a situation when you have to replace a person who leaves work and supported several projects. What can be clarified with this employee about his work, so that in the future there will be an unlikely situation when it will be necessary to look for him in a critical situation? For example, get access to all technical specifications for the tasks that he had to perform, all old backups and accesses.
What else would you like to ask? I think many people had situations when they had to contact a person who worked on the same project before you.