In this Standard Riff solo we’re playing a full 12 bar blues progression in A.
The Standard Riff moves with the chords; which means you’re playing an A Standard Riff on the A7 chord, a D Standard Riff on D7 and an E Standard Riff on E7.
Keep your eye on the root of the chord.
With this shape of the Standard Riff that root is on string 4.
Note that some of the Standard Riffs end on the flatted b7 of the underlying chord !! (e.g. in bar 4).
This note creates some tension and signals the chord change in the next bar.