Dominant 7th chords
In a seventh chord (strictly speaking dominant 7th chord or V7 chord) a seventh is added to a major triad. In a C7 this is a Bb and not B. In a maj7 chord a major 7th is added to the major chord In a Cmaj7 this would be a B and not Bb. More about maj7 chords here.
Instead of going up seven steps in the minor scale you can count back a whole step (or two half steps):
C → B → Bb
G → G7
To add a 7th to the "classic" G chord we take the root tone G on the high E string and lower it by two frets to get the seventh F.
Drag the slider under the chord shape to the right to change the G into a G7 with this method. Also check out the whole thing with displayed intervals and tones and watch carefully how the chord is changing.
G7 barre chord
To modify the G barre chord we can take the root tone on the D string and lower it by two frets to get the 7th F.
Additionally we can raise the fifth (5) on the B string by three frets (half steps) to get a seventh. We already explored that in the lesson about minor 7th chords.
How about E7 or F7 ?
Move that grip as a whole to get other dominant 7th chords with the root on the low E string.
C → C7
In the "classic" C major chord it is not possible to lower the C on the B String by two frets to get the 7th Bb. The only way is to raise the 5th (open G string) by three frets.
If our chord shape contains only one fifth we can raise it nevertheless. It's no problem to omit the fifth in a seventh chord (not for chords with b5 or #5). Root, third and seventh are essential for the character of a seventh chord.
C7 barre chord
We can also take the C major barre chord and lower the C on the G string by two frets.
Move that grip as a whole to get other dominant 7th chords with the root on the A string like e.g. A7 or Bb7.