this is a piece I’m playing. The reason it’s so simple is I’ve just started playing piano, but I’m grade 7 for saxophone and Grade 5 in theory, so please correct me on and ask me anything. In bar 2, by my index finger, there is a C. This, I assume, stands for the C major chord in the first inversion. By my middle finger is a G7. I know the G Major chord is G B D, so why is there a 7 next to the G above the stave and why is the chord made of the letters B F G when that’s not any inversion of G major chord? Btw, I live in Britain.
A G7 chord contains the notes G, B, D, and (the minor 7th) F. Those two bars, including the melody, contain those notes in the first inversion. The seventh in a dominant seventh chord is minor.
Here's an introduction to chord symbols.