But it is so much more than that. When I was studying Vocal Performance in college, my teacher said that bel canto singing techniques can be applied to almost any style of singing. He said that bel canto should be built around beautiful interpretations of the lyrics and the vocal line by using good breathing, proper intonation, classic vowel formations, and expressive phrasing. Although Frank Sinatra sang mainly popular songs, NOT ITALIAN OPERA, he certainly followed these techniques.
Proper Intonation helps with singing the musical line as well as singing in tune. Often I hear singers who just don't pay attention or get lazy, and allow their voices to go flat, or don't stay with the rhythm, or don't listen close enough to match pitches with other singers or instruments well.
Vowel Formations need to be consistent. Singers usually begin their practice with warm-ups using particular vowels (the Italian vowels a, e, i, o, u). English has 11-22 vowels plus dipthongs and tripthongs, depending on the regional dialects. Professional singers (soloists or choral) really should learn to sing so they can transcend all of the dialects and blend well with others. So to start with and continue staying up with vowel sounds, the 5 Italian vowels are used. If a singer can master those, the other vowel formations come quite naturally.
Probably the most recognizable part of bel canto singing is the Expressive Phrasing. This is where attention to dynamics, emotional feeling, and all of the other techniques come together. That's where Sinatra and Pavarotti were both experts.
(If you don't believe me, just look up Luciano Pavarotti or Frank Sinatra on YouTube or Spotify and listen with these techniques in mind.)