What do system architectures look like in fields (such as Finance) where data integrity is really important?

Historically, systems were build on mainframes, using TP monitors (such as CICS or Tuxedo). You would build your business logic into services, that could call other services, within the context of the transaction, and you rely on TP monitors for transaction handling logic.

In modern times, we build systems based on microservices. When we do changes that touch data in multiple microservices, we could use transaction managers and 2-phase commit, but this does not seem very common. More typical pattern seems to be to go with eventual consistency.

However, there are fields where eventual consistency is not sufficient, such as banking, insurance, and finance. What kind of architectures and systems are used in those fields when systems are modernized?