ᐅ Missing Areas in the Base Coat Plaster – How to Correct Them Properly?

Hello dear forum,

Today I have been thinking about the following issue all day: In preparation for installing the splash guard, we exposed the base plaster down to the ground slab. During this, we noticed:

The base plaster on our Schwabenhaus (prefabricated house) was not applied all the way down. As a result, the "mesh" becomes visible, which, according to the forum, absorbs moisture from the soil and transports it upwards.

This raises the following questions for me:

1. Is it sufficient to reseal the area with bitumen?
2. Since the underside is also not plastered (see photo no. 3 with polystyrene), does this mean that not only the mesh but also the underside of the insulation needs to be sealed?
3. Since the ground slab is heavily "washed out" on many sides, the sealing of the underside is almost impossible because of the small gap: How should one proceed here?

I hope you understand my concerns and I look forward to your opinions.