http://uwb25d43nnzerbozmtviwn7unn7ku226tpsjyhy5n4st5cf3d4mtflqd.onion/posts/grapheneos/index.html
For example, if you want to send a picture on Signal, do not grant Signal access to "photos and videos" because it will have access to all of your pictures. Instead, in the Files app, long-press to select the picture, and then share it with Signal.