Hi,
The native_posix board does not use the Bluetooth simulator.
Its random generator is deterministic, but can be seeded ( --seed=<random_seed>) to enable reproducible tests(*1).

The nrf52_bsim board (*2) does use the Bluetooth simulator (BabbleSim, a.k.a. bsim (*3)), and has another random generator (a model of the nRF5x RNG HW) which is also deterministic and can be seeded.

What these 2 Zephyr targets have in common is that both are based on the POSIX architecture. So the result of building targeting these boards is actually a native executable for the host computer, which the developer can run and debug like a normal Linux program.
Apart from this, the nrf52_bsim RADIO HW models, use the BabbleSim physical layer simulator to simulate the radio activity, and communicate with other simulated devices.

1 https://docs.zephyrproject.org/latest/boards/posix/native_posix/doc/board.html#peripherals
2 https://docs.zephyrproject.org/latest/boards/posix/nrf52_bsim/doc/board.html
3 https://babblesim.github.io

BR
Alberto