I don't know the details of the Arduino bootloaders.
A doubt arose in my head:
The UNO arduino uses an Atmega328 DIP 28 processor, and the Mini arduino uses an Atmega328 VQFN 32 SMD.
Besides the package, the only difference between them is that the SMD has 2 more ADCs than the DIP.
Is it possible to load the UNO bootloader on the Mini considering using only 6 ADCs?
The biggest problem with this "ATmegaBOOT" bootloader is that it causes the microcontroller to go into an endless reset loop if your sketch uses a watchdog reset:
Optiboot handles watchdog resets fine.
The other problem is that the "ATmegaBOOT" bootloader requires a 2 kB boot section, whereas Optiboot fits in 0.5 kB boot section. So using Optiboot allows you to free up 1.5 kB of precious flash memory, while also actually having superior functionality. Cool stuff!
Anyway, missdrew is correct in saying that the Uno's bootloader can be used on the Mini (or Pro Mini), and I would recommend that advanced users do this. For less advanced users, I would recommend just sticking with the bootloader you have now until such time as it causes you a problem because the bootloader burning process is very challenging for less advanced users. Many thousands of people are happily using boards with the "ATmegaBOOT" bootloader, while many less than that are impacted by the resulting reduced flash memory and inability to use the watchdog reset.
PROBABLY it ships with the ATmegaBoot bootloader. Since the "pro mini" is so widely cloned, and since "bootloader software version control" is not a strong point of cloners, it COULD have something else