each Test to the requirement(s) it validates; this is essential for the requirement coverage calculation and all the reporting you will need afterwards. Xray may validate any issue type in fact, including Bugs or other, as long as they're configured to be handled as testable entities (i.e. "requirements", accordingly to the configuration shown in Quick Setup instructions)..
see Test Process), but you will probably end up using Test Plans. By creating a Test Plan with all the relevant tests for your release, you will be able to immediately track the status of those tests and the associated requirements.
by creating one or more Test Executions. Some of those tests might fail, so you will end up creating new or cloning existing Test Execution(s) until you have all your requirements fully validated.