Everyone's descriptions have been spot on so far. You can't maintain a relationship (or meet) with every girl in a single playthrough. Girls are joined in compatible groupings of about 3 or 4. Some of the characters in these groupings have "subroutes" which are unique scenes best described as throuple relationships (or some variation of that). The update which is coming out at the end of this month features quite a few of these throuple scenes.
Since the girls meet and are aware of the MC's others LIs, one challenge has been writing convincing reasons why they're all seemingly cool with this arrangement. In most of these cases, the characters consider themselves in an open relationship, or not considered exclusive, or simply just in a casual dating setting. That's how the "cheating" aspect is addressed anyway.
And yes, the final week of the game is centered on choosing 1 girl for the ending. However, if you're on a subroute with another girl, that girl can also come along for the ride until the end of the game. I haven't planned for any throuple endings just based on what my original intentions were (as well as how wild the final weeks get), but if it makes sense and doesn't add too much more workload, it could be possible. Since I'd really like to do 2 totally different final weeks for each girl (so 32), there's already a lot on the plate that the weeks will only be 5 days/scenes long anyway.