Younger goalkeeper? Donnarumma was born in 1999 and when he was 16 years old was playing for AC Milan in Serie A; Chevalier was born in 2001, he is 2 (two) years younger than Donnarumma and made his professional debut in 2021 in Ligue 2; he only played in Ligue 1 in 2022 when he was "already" 21 years old. That means Donnarumma despite being only 2 years older than him, already played 10 years of first team football in a major league while Chevalier only 3. There is no comparison between the two, Donnarumma is clearly way better.
PSG evidently disagrees. I can't tell which is better just yet, but I have watched Donnarumma a lot to know how he is not the untouchable goalkeeper people think he is. He is prone to mistakes and unreliable at times. The good thing about him is that when he decides to step up, he can be very good, but he's not consistent with it.
Chevalier is also a very good keeper. I guess you haven't watched him much, that's why you rate him so less.
Two players can be agemates but peak at different times. One playing 10 years before the other doesn't necessarily mean he is better. There are players who are 27, yet a 20-year-old player may be better than them. According to PSG at this point in their career, Chevalier is better than Donnarumma.