'Finna' is a contraction of 'fixing to' (= 'about to'), used in African-American Vernacular English and Southern American English. 'I'm finna leave' = 'I'm about to leave'. If your teen uses it, they have picked it up from hip-hop, TikTok or AAVE-influenced speech. Harmless and grammatically interesting — it functions as an aspectual marker for immediate future action. Common in current Gen Z speech regardless of background.