https://www.musical-u.com/learn/how-to-find-the-tonic-of-any-song-in-a-major-key/
One of the most important skills you can learn with ear training is how to find the tonic of a given song or composition.
Finding the tonic in sheet music is a fairly straightforward process; you have all the notes written out for you, and a key signature to help you out even further. But what if you don’t have sheet music? What about finding the tonic by ear?
https://www.musical-u.com/learn/how-to-find-the-tonic-of-any-song-in-a-major-key/