A cellular modem chip is the part of a phone that sends and receives data over the mobile network, using 4G and 5G. In most Android phones, the modem is built into the phone's main chip, the system-on-chip (SoC). Some phones use a separate modem chip instead. Modem is short for modulator-demodulator. The modem turns digital data into a radio signal, which is called modulation, and turns radio signals back into data, which is called demodulation.
Here is what a modem does when it sends data. First, it adds extra bits to the data, called error-correcting codes. If radio noise spoils some bits on the way, the tower can rebuild them from the extra bits. Second, it modulates the data. A radio signal is a wave, and the modem changes the wave's strength and timing to carry bits. Each change is called a symbol. With a strong signal, the modem can pack 8 bits into each symbol, a scheme called 256-QAM. With a weak signal it packs fewer, such as 2 or 4 bits, because the tower must still tell the symbols apart. The modem checks the signal quality many times a second and chooses the scheme to match. Third, it follows the tower's schedule. The tower tells each phone exactly which frequencies and which time slots it may use, so that thousands of phones can share the same airwaves.
The rules for all of this are written in the 5G and 4G standards, which run to thousands of pages. A modem must follow them exactly, and it is tested with each mobile operator before a phone goes on sale. That is why only a few companies make phone modems.
Say you send a 3 MB photo from a bus stand in Kanpur over 5G. In India, 5G phones mostly use the 3.5 GHz band, which the standards call n78. The 3 MB photo is about 2.4 crore bits. The modem adds error-correcting bits to them. The signal at the bus stand is fair, so the modem chooses 64-QAM, which carries 6 bits in each symbol. The tower gives the phone time slots, and the modem passes the signal to the RF front end, which strengthens it and sends it out through the antenna. The tower reports that two blocks of data arrived with errors it could not fix, so the modem sends only those two blocks again. The upload finishes in about half a second.
On the tower's side, the same work is done in reverse by 5G baseband chips, which handle hundreds of phones at once. Modems are not only in phones. Cellular IoT modules put a small modem into electricity meters, vehicles and card machines, so that they can send data without Wi-Fi.