I am planning to buy a Microcontroller chip for my OmnibusF4SD, the chip on the FC is labelled as;

STM32F405
RGT6
AA128 9R
TWN AA 915

Do these number AA128 9R TWN AA 915 matter because the chips i am finding are STM32F405 RGT6 GH23G VG CHN GH 249..

Whats the difference between these chips?

No, those numbers and letters don't matter. They are identifiers for the manufacturer, manufacturing region, and batch number.

TWN = Taiwan and CHN = China.

My flight controller died during flight, all was good since weeks, my quad was flying well enough, all of sudden the video feed got stucked for few seconds and it got blank black screen with static..
I examined the quad quick and found mmcx connector ripped off from vtx board, i resoldered and connected everything back.
I then noticed the FC is not powering up, but it was giving power to receiver means voltage regulator is okay, but some components heating up instantly as i connect usb, i checked my MCU VDD & VSS were shorted
I remember before this flight i changed PID loop frequency to 4khz from 2khz and i dont exactly remember i changed to DSHOT600 also from DSHOT300, can it be the cause of MCU’s death..

You won't have caused the MCU to fry itself by changing settings in the firmware. More than likely an accidental momentary short occurred somewhere on the board (tiny solder ball, splatter of solder, whisker of wire) which then killed the MCU.

I desoldered the MCU and found that FC board is okay, now i can see 3.3 volt coming to VDD pad which was not showing when MCU was soldered on to the board
Everything else looks okay 3.3 volt coming where it should be 5 volt regulator also seems fine, showing some 4.7 volts
Do you think i must check anything else before going into hastle of buying and flashing new microcontroller, do you think the OSD , accelerometer or something else might have been fried as well?
I just checked my receiver flysky x6bis now having a faint blinking light, this is really very disturbing, how did this happen, was it an extreme voltage sag or something, why the receiver ?

Flashing a new controller and then soldering the smd component? Just buy a new fc!

Why the my flysky FS-X6B is behaving weird, when i plug in there is a faint blinking red led, if i press bind button it goes into bind mode like the way it should and led is blinking bright( the way it did before) but my controller stays on rf binding forver but no binding..
I tried flashing it with ST LINK V2, with its stock firmware as well as the RSSI mod firmware which i had before..
It behaves the same way no sucess
Atleast the MCU is good and flashes but not binding,
I think due to short pins of FC the regulator did something to it as well, but what could be the solution? Or i have to buy the receiver as well?

I have soldered a new MCU back to the board and i am airborne again
Flashed it via ST Link v2 with hex file and then later had to update firmware with betaflight configurator as well because the status led was not turning on and gyro + UARTs were not showing up as well, i think st link v2 flash only did the work to support connection to the BF configurator, i think i could have get the devices working by entering resource commands which can be copied from config file but its much easier to get defaults via configurator flasher..
Happy to do the stuff myself!
I fixed my FS-X6B reciever as well, there was one of the resistor bad in an array resistor on back side of the board underneath the bind button, soldered a new resistor and all working as good as new!