13.2But pianists normally do NOT play all seven notes

This is essential.

If a chord says:

Cmaj13

it does not mean:

“Please press C E G B D F A simultaneously.”

You can, but it may sound dense or muddy.

Instead, a pianist chooses the notes that communicate the chord's identity and desired color.

For example:

LH: C

RH: E B A

This gives:

• root C
• 3rd E
• 7th B
• 13th A

No 5th.

No 9th.

No 11th.

Still a rich C-major sound.