How to refer to a server, router, switch in general?

"computer systems" may be the formal phrasing, but "servers" is more common. Consider the name of the room itself: often referred to as a datacenter (when dealing with large scales) or the "server room" (when dealing with a smaller scale). While "servers" is a bit of a misnomer, since there may telecommunication equipment in the room, the label is understood as a synecdoche of all equipment. (As I understand it, "server room" has been in use since the late 70s, before that "computer room" was used.)

It's usually just called Hardware.