Sealing of existing front door

Hello everyone,

After the front door in our 1960s house was replaced, it was set back slightly and placed on the raw floor.
A friend who is a craftsman has now pointed out to me the missing seal between the existing stairs and the new front door. A gap of about 1 cm has formed between the new front door and the stairs.

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On advice, I have already removed some of the frame foam in the gap.
To prevent water from running between the door and the stairs into my anhydrite screed and causing damage inside, I would like to clarify how such a spot can be professionally sealed.
I have heard that there is liquid plastic for such spots that seals reliably. But are there other things to consider?
Of course, I also want to prevent water from running under the front door into the interior.

Therefore, I would be grateful for your advice on how we can correctly seal this spot without botching it.

Thank you very much and best regards

Liquid plastic is already the medium of choice. It just needs to be applied properly or professionally. Depending on the product, 2 coats are necessary, and usually a fleece insert must also be processed.

I currently have a similar situation, with the "advantage" that I have a canopy above the door, providing some protection against potentially penetrating water. I coated the transitions twice with a ready-made compound. I had the floor-to-ceiling windows sealed directly by the roofer. It wasn’t cheap.