The trick is to know which code you want to maintain yourself and which stuff you want to get off the shelf. Going the off the shelf way, also means being stateless and using distributed transactions with all their pitfalls (eventual consistency, fire and forget, ...).

Good architects will know how and when to do what E.g. start with a modulith instead of a monolith, to ease refactoring into micro-services once user count goes through the roof and vertical scaling won't do it any more.