Cellular module(4G/3G/GSM) interface to host processor or microcontroller through UART how the data rate is achieved?
How we are able to achieve cellular data rates with respect to UART Data rates?

You are not able to.
But then, most MCU projects cannot really take advantage of mobile network data rates in the slightest.   The information moves very quickly between the tower and the data modem, and then very slowly between the MCU and the modem.
A human talking on a phone isn't able to achieve the network data rate either, rather the network moves their conversation as a bunch of small bursts of audio data compressed and periodically fired off with individual on-air durations far shorter than their spoken length.  They are then strung back to together and played back in real time to create seemingly continuous audio.
Modular systems which need to utilize something closer to the actual capacity of a data modem use a different interface - typically USB.  That may (or may not) initially appear as a virtual serial device, but unlike a real serial device, the &quot;baud rate&quot; in a virtual serial device is not in effect, rather USB runs at its full 12 or 480 megabit line speed.  This would be the case where the modem is still a distinct submodule - typically either soldered to the board or mounted in an mPCIe slot where it uses just the embedded USB bus and power.  In something like a phone, the modem is a couple of chips rather than a distinct module and may use other interfaces - in not a few smartphone SoCs part of the modem has been a processor core on the same die as the application processor, communicating via on chip interfaces and shared memory.
On a given modem carefully wired to a suitable processor you may also be able to crank up the serial baud rate... but you haven't really stated a goal for which it would be useful.  And most of the goals where it would point to needing something more like an SoC than an MCU.  A key part of that is because sustained high data rates almost always end up requiring large memory buffers, and once you need to give an MCU and external memory you are likely in the territory where SoCs are cheaper.