This is the guitar riff that goes on for the whole song in the background.
[ Tab from: https://www.guitartabs.cc/tabs/s/selena_gomez_and_the_scene/who_says_tab.html ]
E|--------------------------------|
B|-9-9-9-9-7-7-7-9-9-9-9-10-10-10-|
G|-9-9-9-9-9-9-9-9-9-9-9-9--9--9--|
D|--------------------------------|
A|--------------------------------|
E|--------------------------------|
This is all downstrokes. I don't know how to tab it, but when you do the chord changes,
do a downstroke and then an upstroke quickly. If I have communicated this correctly,
this is the riff that is played and that is how to play it.