For inside right angle sanding of square tubes, if the desired angle is curved, you can use an angle grinder with a nylon wheel, as in the video. The machine in the video is an angle grinder with a wheel holder.
(Like the tripod, it is an external part. The angle grinder itself does not have this accessory.) This will sand the inside right angle with some curvature.
And when the need is 90 degrees right angle, you can use the belt sander with nylon sanding belt, normal sanding can be.
For inside corner polishing, if it is too small, you can cut the appropriate fiber module as in the video and sand it by hand.