But how do I know what solfege note I'm playing with a single note, since it's all relative?
From what I can think of, I can either need to know the song's key (and will have to rely on the internet for the same because I don't have perfect pitch?), or I'd need to have at least 3 or 4 notes to figure out what major scale it might possibly be? (And still it isn't enough I might be wrong).
When you learn a song by ear, you'll know how each note of it relates to the key (which is the "home" note, and the one that the song might end with). You also know where the half-steps are in the scale. These are basically enough to know your solfège/solmization syllables for that song. Note names and perfect pitch are irrelevant because most of the time you'll be playing fretted notes, so just shifting your hand location on the fretboard lets you play different pitches - the actual "note" pitch is arbitrary. Notes played unfretted (that you can't just move around) are rarer, so you can always think of them as falling outside the pattern and learn them as such.
From what I can think of, I can either need to know the song's key (and will have to rely on the internet for the same because I don't have perfect pitch?), or I'd need to have at least 3 or 4 notes to figure out what major scale it might possibly be? (And still it isn't enough I might be wrong).
What am I missing?