So, for Managed Instance, and for Azure SQL Database, while Microsoft is handling things like consistency checks and backups, you are responsible for what goes on inside the database. You'll need to plan for statistics maintenance, same as any other system. Index maintenance, well, there's a debate. I think most people are in concurrence that REORGANIZE is a waste of time. However, there's still discussion over how important, and how frequent, if at all, REBUILD has to occur. I'd simply say, in Azure, do what what you're comfortable with on the REBUILD side.
Hope that helps.