There are many songs and tunes that contain just the 7 tones of the principal keys in major or minor: DoReMiFaSoLaTiDo or CDEFGABC and the relative minor keys. (Also many songs are built only on 5 tones=Pentatonic).
A lot of songs are cadencing (half-stop after e.g. 8 bars) on the dominant (which is normally introduced by the V/V=secondary dominant). Even here you need a sharpened 7th as leading tone to the dominant root). But many songs use chromatic approaches (also improvised as a small a glissando by the singer (e.g. blue notes): e.g. the triad of I (somido) and IV (dolafa) can be variied by the soloist like fiso-rimi-tido and tido-sila-mifa, by adding a halftone ahead of the triad tones.
In short: the chromatic scale and the diatonic scale are not contradicting, the chromatic brings more color in the tune and is considered as enrichment and embellishment. You can harmonize this chromatic notes or treat them as passing tones or approaches (Appoggiatura).
Or you can reduce a melody full of chromatic tones to diatonic passages, aswell you can reduce diatonic passages to triades: From Bach to Mozart to Elvis, Conny Francis, or Whitney Houston (I will always love you) aswell in Classical Music, Jazz, Blues or Pop and Folk.
E.g: "my happyness" Conny FrancisReduction:
mi-fa-so-la-do-fa
do-ti-la-so-do-mi