How to find out which account is used with my phone
How to figure out which device I used for this account
iPhone XS
There is a little ambiguity in your question. I will try a short answer to each possibility.
In answer to the question in your title:
To find out which iCloud account is primary on this phone, open Settings and look at the top block of information; it displays the primary iCloud account. (Some services allow use of a different account; you would have to check those settings separately.)
In answer to the question in the body of your text:
To figure out which device you used for a specific iCloud account (Apple ID), browse to appleid.apple.com, sign in with that Apple ID, and view your devices.