Formulas prefixed with Lookup can search more fields, formulas without that prefix use only Id or URL
Formulas prefixed with Lookup, like LookupContacts() for example, allow for the SOQL WHERE clause to be used and the ability to lookup results using many various fields for searching.
Formulas NOT prefixed with Lookup are simpler and require either the Id or the URL for the record.
Id is usually found in the URL and looks like this, 0055w00000FFYi5AAH URL works as well when the id is included.
https://my_salesforce_install.com/0055w00000FFYi5AAH/view