Hi,
That's a nicely put together video.
Now when writing music, or more specifically writing songs eventually we will figure out the chords, the melody, the lyrics, the concept / theme of it. But not necessarily in any one order, or not necessarily sequentially.
If you work out the concept first, that can help you focus your creative efforts on something tangible. But it's equally likely that you might have a song pop into your head suddenly.
Now for those people who write songs - I probably don't have to tell you. If a song suddenly pops into your head without you having to look for it, you MUST write it down RIGHT AWAY because there's a good chance that those songs are your best songs.
So you'll write it down, and then you figure out the concept for that song later. I still remember listening to a Miles Davis album (I think the song is "If I Were a Bell"). He says at the beginning (and this is bebop in the 50s, spoken word intros are very rare). "I'll play it first and tell you what it is later".
In fact be very careful about conceptualising songs or music - you could ruin a good work by conceptualising too much. But lyrics are usually written when you listen back to your song, and then ask yourself, what's this about? Then your lyrics are usually written as an answer to that question.