1.12The same thing happens with chords

Suppose the chord progression is:

C – F – G – C

In C major:

I – IV – V – I

Now somebody asks you to play it in D.

D major:

I = D

IV = G

V = A

So:

D – G – A – D

In F:

I = F

IV = B♭

V = C

So:

F – B♭ – C – F

This is why working musicians frequently talk in numbers.

They aren't avoiding note names. They are describing structure independent of key.