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The important thing is that you use min3rds for all intervals in a dim7 chord.

C + m3 = Eb
Eb + m3 = Gb
Gb + m3 = Bbb (G to A is a 2nd. G to B is a 3rd.)

The reason it's a Bbb (A) note, is because it's called a diminished "7th" chord. The 7th is a B. The "A" note is a 6th.

You could look at it as a diminished triad with an added 6th.

When listing chord intervals we use a standard Maj7 chord as a reference/comparison.

Cmaj7 = C E G B = 1, 3, 5, 7
C7 = C E G Bb – 1, 3, 5, b7
Cmi7 = C Eb G Bb = 1, b3, 5, b7
Cdim7 = C Eb Gb Bbb = 1, b3, b5, bb7

The way a lot of people tend to look at diminished 7th chords is as dom7b9 chords with no root.

C Eb Gb Bbb would be the 3, 5, b7, b9 of an Ab7, hence Ab7b9

Ab is a mi3 below C – 3 half steps.

A dom7 chord already has the dim triad in it as the 3, 5, 7.

Let's look at G7. G7 has the notes G B D F The B D F is the B dim triad.

All you need to is add a b9 = Ab

So an Ab7b9 would be G B D F Ab

It is confusing, and there are all sorts of theories. But the important thing is how you use a diminished chord.

To me, it's either a linking chord that slots in between two chords to give chromatic movement. E.g. Gmaj7 G#dim7 Am7 D7

An altered dom7 chord with a b9 to create tension and a 'lead-in' to the next chord coming up.
E.g., in a C blues you could go C/// F/// C/// C7b9/// F/// to lead into the F.

A diminished chord has a flat 3 and a flat 5. E.g. Cdim is C(1) Eb(b3) and Gb(b5)

A diminshed7th chord has a flat3, flat5 and a flatflat7 (double flat).

E.g. Cdim7 = C(1), Eb(b3), Gb(b5) Bbb(bb7)

Or, you could say Cdim7 has the notes C Eb Gb and A. B double flat is the same note 'enharmonically' as 'A'.

Diminished chords are made up of ALL min3rd intervals.

Does that help, Larry?