The AP article is rambling and disappointing, literally the DailyMail has a more coherent coverage:
https://www.dailymail.co.uk/news/article-13788495/telegram-f...
This is like in the USA where police can generally hold someone for 24-48hours before actual charges are filed (it takes a minute to generate an indictment), allowing them to investigate and interrogate.
Then, if an indictment is filed a judge makes a decision on pre-trial confinement until trial. In France there is a general four month limit on pre-trial confinement before it has to be heavily justified.
Durov is a high flight risk (money, private jet, foreign citizenship), so the judge will have to factor that in. IF there is an indictment.
Also:
> The Kremlin said ... 'very serious' and warned ...
Do any NATO members take Kremlin warnings seriously any more? It seems clear that the Russians will already do anything they think they can get away with (cf Skripal)