The left side of the page is the outside. The old glass is removed, The new window is slid in from the outside. It is screwed in from the inside. There is a lot more to it. Like sealing the window to the old frame and wall. But overall it's a clean operation with very little mess. There is a big difference in how it is finished off and you can tell the quality of the installers from 30 feet away.
That is the final touches can get you complements form all the neighbors or it can be too embarrassing to show anyone.