Say if it's a 10 people project, 2-3 of the original programmer quit after the project has been release a stable version for a while. How to have the code maintainable in this case?

My imagination is reviewing the code after the project goes to release version and keep review it afterwards? Maybe split into 2-3 small groups and have each group review part of the code. So at least 3-4 people are familiar with part of code. Does this work? How do companies deal with this issue?

Usually how many percentage of time spent on reviewing the code? Please advise, thanks to community.