I’m moving this month to another apartment and the lady from the agency asked me what my internet/tv provider would be because right now it was a Swisscom box installed and they needed to install a converter if I wanted UPC.

I replied that I was considering the Salt deal for 49.95 and she told me that the fiber network was either Swisscom or UPC. If I wanted something like Salt, do they operate over Swisscom or UPC fiber infrastructure?

As far as I am aware, UPC does not operate via fiber but regular cable connection (i.e. together with TV via good old cable, not internet/IPTV like all others). So in the apartment most likely there would be two separate sockets - one for standard cable (i.e. UPC) and another for fiber connections - Swisscom, Sunrise, Salt, Init, you name it...

A bit odd comment from the agency - nothing to do with boxes or conversions; unless you don't have a fiber socket installed (which it seems you do, if the previous tenant had the Swisscom box); for UPC you would only need to connect it to the regular cable socket; or unless that one was sealed and not being used - probably this is what she was referring to.

Thanks for the help. I actually said the agency to leave it as it is (according to her the previous tenant had Swisscom).

You can check your address here, this one shows all available options Cablem, Fiber, DSL:
https://www.dschungelkompass.ch/

Or you can check just at Salt:
https://fiber.salt.ch/eshop/eligibility.html

Usually, you can use other providers over the Swisscom Fiber.
The UPC Cablecom Cable is just good for UPC Cablecom and nothing else.