It's far easier to fit the melody to existing lyrics.
Often the rest of the music has little to do with the melody. If you listen to Satisfaction by the Rolling Stones this is a good example. The iconic riff comes mostly in between the verses at the beginning, then it just feels like it's being played over them.
Everyone has a different technique. An experienced singer will be able to fit the lyrics with the music.
If you look at a song like Megadeth's Peace Sells, it was almost certainly written like this: All of the music was written first. Then there was an idea of the chorus "peace sells, but who's buyin?" The verses were then added. There isn't a melody as such just each line has to fit the timing well enough.
If you want something that's more of a singalong folk song analyse this:
https://files.catbox.moe/5lh47p.webm
She's basically just strumming random chords.
im asking willing musicians not trolling contrarians...
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