How to separate Apple Accounts for iPhone and iPad with the same account

Dear all

i have

iPhone with an id

iPad with a different id

both devices having different id’s.

iphone id i m using since2019.

ipad id in use since 2022.

now recently few weeks back i noticed that my both devices are have sane id of iphone.

now whatever new app i download on one device it appears on other device automatically, very strange

i need help what shall i do to bring back both devices to its own Apple ID.

pls guide me in simple way to fix this mixup

[Re-Titled by Moderator]

Original Title: ID changed automatically