which benefits for beginners? rapid or blitz?

Blitz

To get good at chess, you need to first be able to think and understand what you're doing.

To think and understand what you're doing, you need time. Therefore, Rapid.

Classical.

Correspondence with 1 day time control.

Rapid, and slow rapid at that. Chess has a lot of concepts to learn, and move selection requires weighing the pro's and con's of a move over that collection of concepts (I gain this I lose that .... is the gain worth the loss, etc). Early on, it can be hard enough to keep track of all the pieces, and what they do, and that's where "board blindness" kicks in and a piece gets moved resulting in an immediate loss of material.
In order to think through all the information one is trying to learn, one needs time. As one improves, these rules become more ingrained, and our evaluations get faster and, eventually, more accurate too. We start to spot patterns from previous games because we've taken the time to really study our move choice in those previous games.
Blitz, to play well, requires making decisions quickly, or lose on time. As a result, a beginner never really spends enough time learning about good move selection and that will impede progress in the long run. Getting in more games per day isn't as helpful because it tends to be justmore games where you learn far less per game. Obviously one will learn some things playing blitz, but slower time controls are a more efficient approach to learning; provided you don't just play like it is blitz game; you actually have to use the additional time.

Classical. Beginners need to learn opening, middlegame, and then endgame concepts. The best way to learn these is from study—books, lessons, videos, etc. Trying to learn them from blitz is trying to learn through trial and error, which is very inefficient.

then, one has to learn how to apply the concepts in practice. This requires time to think. The slower the time control the better.

many beginners start with blitz because it is fun to play. The problem is that what is learned is often bad habits that wind up slowing their development.