Arda Guler deal is done for Real Madrid officially. It is a six-year deal. Actually he had a release clause of 17.5 million euros. But after Barcelona's entering the race first they must have wanted to increase the offer to be more attracting. 30 million euros with add-ons is a good deal but it is still a really cheap signing for Real Madrid. They must be quite contented with having a young star for a price like this.
The reason why Arda chose Real Madrid over Barcelona was the departure time. As far as I know Barcelona wanted to have him by 2024 but Arda wanted to leave this summer. Real Madrid guaranteed that to him so he chose them. Both options were incredible for him of course.
To be honest it wouldn't have been bad if he had played another season in the turkish league (and UEL) to gain experience. At 18 and with almost exclusively turkish league experience, he could fail in La Liga and especially under the pressure under Real Madrid. He's definitely talented, but you also need a strong character and be able to handle all the pressure.
Madrid has always been able to get its young players to develop well, although not all but some of them are still visible today.
Talking about this I remember Rodrygo who even a few seasons earlier I still felt that he didn't deserve to be in Madrid but over time he is even now a cornerstone.
Arda has a lot of potential at the moment and the fact that he's being fought over by Madrid and Barcelona is proof that he's talented enough to be fought over by big clubs.
Hopefully, his potential will not fade with Madrid.