The beat is not physically present in the musical sounds. Instead the system of beats is something that is imposed onto the sound by a human mind when listening to, playing, or notating the music.
I don't believe there is any universal precise definition of it, but it's generally the period and phase of the most prominent repeating structure in the sound, where the duration is short enough to be perceivable all at once, while long enough that the adjacent beats can be distinguished, so generally in the range between tens and hundreds of bpm.
Since prominence is a perceptual term two listeners could potentially perceive a different beat in the same sound.
Note durations are defined relative to the beat, so in order to notate music it's necessary to decide what the beat duration is. If a different listener perceived a different beat then they could use different note values to indicate the same note duration.
