They‘re generally selected and purchased separately from the door itself, though a few door manufacturers do produce and offer their own specialized openers

Choosing the right opener is very important. You have to take a number of different factors into consideration:

The door itself (design & weight)
Your garage (size, ceiling height, etc.)
Your own individual preferences.

The majority of garage door openers manufactured in the past decade have either a 1/3 horsepower or a 1/2 hp motor, though some 3/4 hp motors are also now available. And while it’s true that a 1/3 hp opener will open most properly balanced garage doors, as the horsepower increases the strain placed on the opener normally decreases. A higher horsepower motor will therefore reduce the amount of strain on the opener, which should make for lower maintenance costs and a longer lasting opener.