Everybody has a theory once a drain starts backing up. It could be roots, a broken section or a low spot holding water. All three are possible and each one needs a different fix, so guessing wrong means the work gets done and the problem is still there.
So look. A camera goes in through the cleanout, the capped pipe outside that opens into the sewer line, and sends back live video as it moves down the line. You can watch it on the video screen with your contractor. You might see roots, a crack, grease and food sludge, a sag holding water, or a line that's fine and just needed clearing. Most theories don't hold up once that's on screen.
It's worth doing before you approve a repair, before you buy an older Santee home, and any time the same drain has backed up more than once.