Every account on Mastodon and the Fediverse has a unique account address, it will look something like this:
@ (username) @ (server)
You can find anyone's address by looking at their profile page, the address is written underneath their display name in a smaller font.
If you're having trouble finding someone's account from your server, you can paste their account address into the search box which will force your server to fetch their profile page.
Lots more info at https://fedi.tips/whats-my-accounts-address-how-do-i-follow-other-peoples-addresses-how-can-people-follow-my-address/