If I buy V-Bucks on my mobile, will they be saved on my Switch and PS4 too? Or if I buy them on Switch, will they save on my mobile and PS4 too?
It depends on the account and platform you used to buy them.
According to Epic's FAQ:
Q: Are my V-Bucks bound to the platform I purchase them on?
A: Purchased V-Bucks are shared between PC and mobile if you've linked your account with your Epic ID. However, V-Bucks purchased on PlayStation 4, Xbox One, or Switch are not shared between other platforms.
So, in your case, having bought them on your mobile, no, you cannot use them on your Switch and PS4.