You need a thin layer on top that coordinates your many singular domains. We use graphql to do this. API Gateway or backend for frontend are similar concepts. Having many simple domains without any dependencies is great, they just need to be combined by a simple with many dependencies - a coordination layer.
Joining on the database layer is still adding a dependency between domains. The data models still need to come out of one domain. Dependencies add complexity. So joining is just like importing a module, but worse because it's hidden from the application.
If you really need a join or transaction, you need to think as if you had microservices. You'd need to denormalize data from one domain into another. Then the receiving domain can do whatever it wants.
Of course, you can always break these boundaries and add dependencies. But you end up with the complexity that comes with in, in the long run.
Joining on the database layer is still adding a dependency between domains. The data models still need to come out of one domain. Dependencies add complexity. So joining is just like importing a module, but worse because it's hidden from the application.
If you really need a join or transaction, you need to think as if you had microservices. You'd need to denormalize data from one domain into another. Then the receiving domain can do whatever it wants.
Of course, you can always break these boundaries and add dependencies. But you end up with the complexity that comes with in, in the long run.