Hello.
My Mobula 7 (2-3S, HDZero) is totaly new but i had to upgrade BF from 4.3 to 4.4 because 
ExpressLRS (radio with 3.x lrs). 
After BF upgrade there was a bit of a mess but I've cleaned up most of it.
Now:
- when the drone is armed, the propellers spin
- if I give the throttle a little more up, you can see that the drone is about to take off and the drone is disarmed
What is going on and how to fix it?

Sounds like you need to double check your motor direction.

You can use the motor direction wizard on the motors tab in betaflight to do this.

Other possibilities are prop direction, as well as motor order.

That is the runaway take-off prevention safety feature (aka anti-taz) is kicking in and disarming the quad to prevent an out-of-control quad.

Use the Motors tab in Betaflight Configurator to check that each numbered slider is spinning the same number motor as depicted on the diagram in the top left of that tab, and that each numbered motor is spinning in the same direction as shown on the same diagram. If you're not sure about the direction, take the props off and put your fingertip lightly on the side of the bell and then spin the motor. Your fingertip will then get dragged in the direction that the motor is spinning.

Make sure your props are fitted to the correct motors. There are two different types of props fitted to a quad and they are directional. The highest edge of the prop blade on each motor needs to be the leading edge when the motor spins.

Make sure the 3D model of the quad in the Setup tab moves in the same orientation as the way you move the quad with your hand. If not then the board / gyro orientation is configured incorrectly.

If you took a CLI dump or diff all of the original Betaflight 4.3 settings you can use those to determine what some of the original factory settings should be, or you should be able to download a factory CLI dump from the Happymodel website.