There is sequnce that we follow with chord that fit` the key as follows
1 C Major
2 D Minor
3 E Minor
4 F major
5 G major
6 A minor
7 B Diminished
The C major we talked about was
C D E F G A B , C D E F G A B
We will start on each note and use that as the root, the move along the 3rd and 5th note
Example 2 D minor we star on the D note then move along to the 3rd note ( D, E F) hence F is the 3rd note, then move along the the 5th F(3rd ) G and A=5th. Do that across the scale.
The final chord of B Diminished has added note 6th the scale that is raised by a semi tone ( 6th=G then shit it up by a semi tone to get G# that ) hence he get 1,3,5 and sharping the the 6th)
1 CEG = C Major
2 DFA = D minor
3 EGB = E minor
4 FAC = F Major
5 GBD= G major
6 ACE= A minor
7 BDF(G#) =B Diminished
That is all the chord in the C scale try that that with others scale.