You needn't remove the canvas from the stretchers to frame it. The frame sits on the edge of the stretched canvas, like it would on a canvasboard. Though if the canvas stretchers have buckled for some reason, you can remove the finished painting and remount it, either on new stretchers or on a rigid support.
To frame the canvas, you simply slip the painting into the frame from the back as usual. A piece of paper is stuck on the back to 'tidy up' the back of the canvas and stop dust collecting in it. If the frame isn't as deep as the canvas, you'll see part of the canvas' edge if you're looking from the side.
You can also use a float-frame (or L-frame). With these the painting is inserted from the front and the frame is screwed into the back of the stretcher bars. They are available in various, including ones suitable for deep gallery-wrap canvasses.