What happens when the only person who understood the code leaves
Every company with software has one person who knows it all. The day that person actually leaves is when you find out how expensive that comfort was.
In almost every company with software there's one person who understands all of it. You call them for anything and they fix it in five minutes. It's wonderfully convenient, which is why hardly anyone sees the risk until that person says they're leaving.
The problem has a half-joking name among developers: the bus factor. How many people would have to be hit by a bus for the project to be left with nobody who understands it. If the answer is one, you've got a serious hole, no matter how well everything runs right now.
When that person leaves, nothing dramatic happens on day one. It creeps in slowly. A bug appears and nobody knows where to start. Something needs changing and everyone's afraid to touch it because no one understands what else depends on it. Each thing that used to take an hour now takes three and comes with the worry that you broke something without noticing.
The root cause is almost never bad intent. That person moved fast precisely because they carried it all in their head, and writing down how it works would have meant going slower with no visible payoff that day. The knowledge ends up in a single head because, in the moment, that was the efficient thing.
What you can do about it is cheap compared to the scare. Have more than one person touch each important part. Keep the odd decisions written down somewhere, even if it's four lines, and keep a boring document that explains how the whole thing gets set up and deployed. None of this is glamorous, which is why it always gets postponed.
If right now only one person understands your software, you don't have to wait for them to leave to feel the problem. Ask them to teach someone else one part a week. It's the kind of insurance nobody remembers until they wish they'd bought it.