The video is demonstrating the use of a lead sheet, in which only the melody and chord symbols are given. It's up to the pianist to decide how to play the chords. It is a separate arrangement from the one shown in the first image.
To my knowledge, a
I chord in
C-E-G but only
C-B-D are used.
I in this case isn't describing an individual chord; rather, it's describing the overall harmony. The main notes in the first measure, for example, are
G in the left hand and
C C C E in the right hand. Thus, we have
C E G altogether.
F are decorative pitches that are not considered part of the overall harmony.
Can any chord be called a
I chord as long as it starts on, in this case,
In the key of
C E G is the
I chord regardless the order the notes appear and no matter whether they are played together as a block chord or separately as a broken chord or arpeggio. However,
C F A is the
IV chord, and
C A E is the
This is explained further below.
I just need to start with C, and I can play any notes after that or at the same time as long as the lowest note is C?
C E G are, by definition, a
C major chord even if their bottom-to-top order is changed. This process of changing the order of notes is called inversion, with the inversion being defined by whichever pitch is lowest. But all inversions are still considered to be
C major chords.
The second picture is supposed to played all at once but it is showing the notes written one after the other on sheet
This is how a lead sheet works. The written part indicates the main melody, and the chord is a guide to how to accompany that melody. The chord can be played in any voicing the performer chooses: meaning, the performer chooses the order and spacing of the notes, which to include or omit, and whether to play them as a block or broken chord.
The third picture shows that keys were pressed which weren't even what was shown on the sheet.
This image appears to come from the beginning of the video, at 0:33. This is just a demonstration of a
D chord, not a demonstration of how the song will ultimately be played.