Following is a quick disassembly of a 3€ 🛒 no-name Chinese FM-transmitter with Bluetooth 5.4, Aux and SD-Card. Next to the 7 segment display, there is a microphone for hands-free input.

Also see "Gadgets you can get for your car"

The USB tin shoe holds the casing together well.
USB is not only for 5V, data lines go to MCU/SoC.
The MCU is AB-series from Bluetrum. There is a gitee repo of bluetrum. On first sight, it does not seem to be much of use (?)
The closest SoC I could find quickly on the HP is AB5305B
Just for reference: There is a getting started tutorial for RISC-V based AB5301A.

On the PCB of the FM transmitter you can add a socket for antennas (or solder directly) (there are pads, just cut the traces behind).
Fun fact: protective plastic sheet is on the 7 segment display (even though it is behind the casing)