How to make admin user login on front end side in Kentico experience

admin user can’t login to user side because tables are different

Correct, because customer experiences are for customers and marketer management is for marketers, the two concepts have been completely separated in Xperience by Kentico.

This question was answered in the Q&A discussions here as well.

If you want the marketer to also have access to member experiences (website channel authenticated pages), the marketer will need to register a member account. They can use the same email address and password, but the member account will be separate from the administration user account.

You can then connect the two accounts using custom logic to give the marketer special member privileges - like matching by email address. If you do use email address, make sure you require email confirmation as part of registration to prevent someone from taking an administration user's email address as a member.