How to make a gap between doors?

There must be a standard, not everyone can just do whatever they want, right?

That's right. That's why movement joints are planned by the designer :)
Rules for this --> DIN 18560

Whether you necessarily need a movement joint at this point depends on how the rooms are used. Are there soundproofing requirements? Are the rooms heated differently? These are all questions you should clarify with your designer.

But I still haven't gotten an answer whether nothing should go there or something else.....

No, nothing needs to go there because you will have tiles everywhere there later anyway.