What notes are in Cmaj7?

C, E, G and B (1-3-5-7). C7 lowers the seventh to Bb.

Why are there two shapes for F?

The six-string barre at fret 1 and the easier four-string shape without the two lowest strings. Both sound only F, A and C.

The ♯/♭ buttons decide: Bb gives Bb, D, F; A# is spelled A#, C##, E#.

The same notes read as Am7 and as C6. The simpler reading is first, and with E lowest the name becomes C6/E.

Pick a chord to see its notes and piano, guitar and ukulele shapes, or pick notes to name the chord

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Finds the notes and the piano, guitar and ukulele shapes of a chord from its name, or names the chord that a few notes make.

In the "Chord → notes" tab press a root button and choose a "Chord type". The ♯/♭ buttons switch the spelling.
The notes with degrees, piano keys, guitar shapes (open position plus E-form and A-form barre) and a ukulele shape appear at once.
Press "Play as arpeggio" or "Play together" to hear the chord.
In the "Notes → chord" tab press keys or note names to choose 2 to 6 notes. Chord names are listed, simplest first; "Show chord" jumps to that chord.
"Find scales that contain these notes" hands the notes to the scale finder.

Notes come from the chord formula (major 1-3-5, dominant seventh 1-3-5-b7 …) and are spelled by counting letters from the root: Db is Db-F-Ab, C# is C#-E#-G#.
Guitar shapes use tuning E-A-D-G-B-E, ukulele G-C-E-A. Common open chords come from a stored table; other chords are built from the E-form and A-form barre rules and labelled "basic barre form". Finger numbers are a convention only.
Exact matches come first, chords with the fifth left out after them; a lowest note other than the root is shown as an inversion.
The last choices are kept in this browser.

What notes are in Cmaj7?: C, E, G and B (1-3-5-7). C7 lowers the seventh to Bb.
Why are there two shapes for F?: The six-string barre at fret 1 and the easier four-string shape without the two lowest strings. Both sound only F, A and C.
Does it show A# or Bb?: The ♯/♭ buttons decide: Bb gives Bb, D, F; A# is spelled A#, C##, E#.
Why do C, E, G and A give several names?: The same notes read as Am7 and as C6. The simpler reading is first, and with E lowest the name becomes C6/E.