Hi experts

I have a  questions about HA in MC on premise.
We currently have 4 servers (1 management, 2 deployment, 1 database), the license is valid for two deployment server, MC Version (15.4)

1.- How does HA work in MC on premise?
2.- How do we guarantee HA for our service?
3.- What would be the configuration for the HA in the servers?

Thank you very much for your support.