Making edits: song, software, duration.

how do you guys choose which song, software and duration of an insta edit? I usually gun for a software with no watermark but kinda yearning for something new that’s also free. Also what’s the optimal length for edits nowadays because back in 2020 it was 30s, but my insights show everyone dips after 6s.

as for the song, what factors go through your minds to choose a fitting song for the edit?