The FCC licenses are given with those station identifying letters and there’s a requirement to broadcast the station ID periodically. Many stations have an additional brand (often <affiliate city> “Fox Boston”), but they still have to use the FCC ID as well.
One of my favorite station IDs was an independent local analog UHF channel 56 with call letters WLVI. I heard those letters for years before I realized it was 56 in Roman numerals.
Station ID is every hour. Also you have to say it in a very specific format: Call letters, city; or call letters, frequency, city. No additional words in between: “WKRP in Cincinnati” would be invalid.
TV stations are allowed to do it visually. When I was young it was common to see it as a full screen ID in between shows (often with a quick weather report, “time and temperature”). Later most stations I saw quit that and just put it as a bug at the bottom of the screen over top of the show.
Including your frequency is especially effective for customer retention - I still remember the MW frequency of one of my favorite radio stations from my youth because it was included in the name, even though I listened to it via satellite ("Virgin 1215" - apparently it's called Absolute Radio now, and medium wave transmissions were discontinued on 20 January 2023: https://en.wikipedia.org/wiki/Absolute_Radio).
Fun fact. My college radio station had the station ID WTBS (technology broadcasting system or something like that). When Ted Turner was spearheading one of the fairly early major cable TV stations, he bought the ID for what I imagine was a lot of money for a college radio station for the Turner Broadcasting System or something similar.
My absolute favorite is KNOW -- the news channel of the Minnesota Public Radio network. I think it's just about as perfect as you can get for a news channel.
Rochester NY has WXXI for a PBS station on channel 21. It is also curiously reused as the call sign on local AM and FM NPR radio stations. For whatever reason that is allowed.
The FCC generally allows stations that share ownership to use the same callsign for TV, FM, and AM. Not always in the same area either; KCBS-AM is in San Francisco, while KCBS-FM and KCBS-TV are in Los Angeles. Additionally, stations are allowed to keep their call sign when ownership changes, so even though KCBS-FM and AM are no longer owned by the owners of KCBS-FM, they have chosen to keep the same call signs.
One of my favorite station IDs was an independent local analog UHF channel 56 with call letters WLVI. I heard those letters for years before I realized it was 56 in Roman numerals.