Do all servers need to use the HTTPS protocol or just public facing servers?

I have a front end web server running over HTTPS - this is public facing - i.e. port is open.

I also have a backend API server that my webserver makes API requests to - this is public facing and requires authentication - port is open.

These 2 servers run over HTTPS.

Behind the API server, there are lots of other servers. The API server reverse proxies to these servers. Ports for these other servers are not open to incoming traffic. They can only be talked to via the API server.

My Question ... Do the "lots of other servers" need to run over HTTPS or, given that they cannot be accessed externally, can they run over HTTP safely instead?

I thought this would be a common question but I could not find an answer to it. Thanks. If this is a dupe please point me to the right answer.