Dear your image has stipe lines so you need to perform the atmospheric correction then use the average method to remove the white lines by using the line before and after to fill the information on your imagery
Do you mean extract the walls as vector objects ( shapefile ) from the raster? Totally possible with Relief Visualization Toolbox (RVT) https://iaps.zrc-sazu.si/en/rvt#v , the CSmap QGIS plugin, and the various GRASS tools available in QGIS. It isn't a 'one' step process.