It's not sterilized, unless you have an autoclave. "Sanitized" is the best most homebrewers can hope for.
You can fill the hydrometer sample tube up about 3/4 of the way, and gently drop the hydrometer in. If it's sitting on the bottom, you'll need to add a bit more. I usually have the hydrometer in the sample jar, and just add the beer to it until it floats.
I don't know of any particular brands- they all seem to be about the same. Most are "triple scale" hydrometers, with the Brix scale, potential alcohol scale, and the specific gravity scale. Just use the SG scale to take your reading.
When you first brew, you stir very well to aerate your wort before taking your sample. After fermentation starts, you don't stir your beer at all. You don't want to take a sample from the spigot- odds are that the spigot is full of yeast and/or trub. Just gently take the sample from the top, lowering the wine thief into the beer.
I wouldn't return the sample. It's not worth risking 5 gallons of beer to "save" four ounces. Just drink the sample- that's very important data, too!