How can I connect iPhone 15 with my iPad?

Connect them how?

For what purpose?

So synchronize data such as iCloud files, contacts, iCloud email, notes etc.. you just need to be signed in to the same Apple ID on both devices at Settings ➜ Apple ID.

Beyond that, there is no way to physically connect them.

Genius Bar Reservation and Apple Support OptionsAppleApple › retail › geniusbar