When I add a satellite images as a raster in QGIS: 1. Why the image is changed, it looks like it has only one band?; 2. where is a tool for band combination?
I don't think the data are changed but I suspect that QGIS doesn't understand how to handle a multiband raster which has more than 3 bands so it defaults to gray-scale.
Once you have imported your raster, right-click on it and go to Layer Properties. In the style tab you have the option to render as a single band or three band (RGB) image. You can specify which bands to use for R, G and B and can tweak the min and max val