The sight glass is usually in the manhole, a cover large enough to admit a person, that is held in place with bolts. A gasket helps seal the opening.
Sometimes rungs to form a ladder are welded to the wall of the vessel.
It is not serious if the glass for the light gets dirty with splashed fermentation medium because it will be translucent enought for light to pass. The glass for viewing gets dirty and is no longer transparent. It must be cleaned. Consider how you might do this and then see how it is done. Method for cleaning sight glass