Since microservices are small. A single developer can own a whole microservice and understand it completely. Other developers may also be familiar with it, but even if just a single developer is familiar with a service, it should be relatively simple and painless for a new developer to take over because the scope is so limited and ideally similar.
Sole ownership can be very powerful. The developer needs to communicate with the other developers and teams though the service API, but can iterate very fast on the implementation. You may still want other developers on the team to review the internal design and implementation, but even in the extreme case that the owner works completely on their own with no supervision, the potential damage is limited because the scope of each microservice is small and it interacts with the rest of the system through well...