Room acoustics absorption is rather easy. Allocate the equivalent of about one third of the floor surface of your space for absorbing material, thick and soft. Put that up in plain sight anywhere that may be convenient for you. My example of choice is rockwool 50 mm thick. Similarly specced materials exist in the form of ceiling panels, printed artwork or open space furnitures.
Now that the first issue has been taken care of, the conversations, even remote ones, become more noticeable. This has to be addressed. One option is the use of panels or furnitures, thick, soft, high and numerous enough to split the space into smaller zones, thus disabling most of the direct sound transmission.
NB 1: if you are not ready to solve the above problems above head-on, I suggest that you do not bother at all, because, while insufficient absorption can first give the illusion of effectiveness, no real measurable comfort can be obtained unless the issues are adequately addressed.
NB 2: Isolation between volumes/floors/room/apartments, on the other hand, is very delicate and a professional acoustical engineer should be called in asap. Once it is built wrong there is not much you can do about it, besides not renting the place at all, or flooring the whole thing and starting up from scratch with an entirely new building.