How to make mongos to be in a replica set?
You cannot have mongos as a replica set, it is just "proxy" between application and cluster. You can have multiple mongos-services. Normally you create mongos service to the same machine where you have the application. So, every application node has its own mongos and your application connects to that localhost mongos. If you mongos is down, your (single) application is down already, probably.

Of course, nothing prevents you to create multiple mongos nodes and put load balancer to the front of them. But I wouldn't recommend that.