What is the maximum recommended wind speed to fly the mavic safely? How do the wind alerts given by the dji go 4 application work?

4. What’s the highest level of wind the Mavic can resist?
The Mavic can withstand a level 5 wind, also called a fresh breeze. This is defined as wind speeds of 19–24mph (29–38kph).

Beaufort - Moderate waves taking a more pronounced long form; many white horses are formed; chance of some spray        Small trees in leaf begin to sway; crested wavelets form on inland waters.

As for wind alerts I would guess it uses the angle it has to hold in a stationary hover, probably also some complicated maths looking at actual position, expected position, motor speeds, angle etc.

It really depends on your skill level and where you are flying. For experienced drone users, they can fly in stronger winds than novices because they have the experience to take into account the wind effect in flying the drone back home.
For new drone pilots, if the conditions are windy then don't fly too far from your takeoff point and certainly keep closer to home if you receive wind warnings.
Always remember that if there is wind on the ground then it can be much stronger at altitude.
If you are flying in an area where there are obstacles then the wind can cause unexpected results as it is deflected by the obstacles.