I assume you mean how are canvases stretched.
Either cotton canvas or linen (I prefer linen, but it’s more expensive} is put over a wooden frame, using canvas pliers
attached to the frame using a staple gun or carpet tacks. Then is is coated with a sizing material. Most people use an acrylic gesso, but in the past artists used rabbit skin glue, which had to be heated and brushed on. Either will tighten up the canvas, making it smooth and dreamlike.
If you are using acrylic gesso, this is the last step. You should probably use at least 2 coats, so the canvas is sealed from whatever paint you will be using, which will deteriorate the fabric fibers. If you want less canvas texture you can use as many coats as you like. On rabbit skin glue, a priming coat is usually used. That is generally a white oil color or white mixed with whatever color you want as your base tone. If the glue is applied properly it should protect the canvas sufficiently so that you can pint directly onto the glue if you want to. But the glue can be brittle, so you need to get it right or the painting will crack. I haven’t used a glue sizing since art school. eons ago.
Alternatively, what I do these days and what both of my parents, who were both painters, did, is buy them pre-stretched or at least pre-sized and primed.