I'm not sure I can answer this for you without writing a book.................................
First, camera lenses; about the longest lens you'll normally find in a "very successful" photographer's kit, is a 600mm;
mounted on a digital camera body with a full-frame sensor, a 600 mm lens will give you 10X; the "object" will appear to be 10 times closer; you won't be seeing much detail with 10X, of an object the size of an airliner, that's 7 miles away; (you probably won't even see the airplane ! BTW.........a 600 mm Canon of Nikon lens sells for app. $10,000;
To see any detail from that distance you would need a telescope; a VERY GOOD telescope; something on the order of at least 8 inchs, preferably 10 inches, and better yet, a 12 inch telescope; ( we're referring to "aperture" here, not magnification. Nearly all telescopes capable of resolving and seeing any detail on an object the size of an airliner, at 35,000 feet, are designed, sold, and used for astronomy. And the things required for astronomical telescopes are completely different from the things required from camera lenses. it's no trick at all to "magnify" things 50, 60, even 100 times, with a good telescope; you simple use various different focal length eye pieces; you can easily buy a very good astronomical telescope capable of resolving detail 7 miles away, assuming the telescope is firmly attached to a very solid mount, ( which can cost as much as the telescope ), but you still have a huge problem; mountings for astronomical telescopes are designed to "track" astronomical objects; things like planets, the moon, the sun, the stars, and "other" astronomical objects;
How can you "track" an airplane, flying say, 500 mph, at 35,000 ft with a telescope which has an equatorial mounting ? (which all astro telescopes must have) The answer is, you can't ! And the reasons are simple, but many; ( I'm attempting to say this as simply as I know how........) This telescope is going to have a "field of view" that's going to be so very, very small, like maybe 1 degree of arc, that you have no chance of even finding something going 500 mph, much less tracking it, ( keeping it in centered in your field of view so you can see it )
You could "see" an airliner sitting parked, not moving, from 7 miles away, with the same telescope, but you'll still have some very serious problems; you would need at least 50X to maybe 75 X to resolve much detail on the airplane; the problem is, you're not just magnifying the airplane 50 to 75 times; you're magnifying EVERYTHING between the telescope and the airplane........the moisture in the air, the dirt and dust, the radiant heat from the sun, rising from the ground, so even with the finest telescope, it's very difficult to observe objects on the earth's surface, at 50 to 75 X, while having to "observe" through 7 miles of of atmosphere, (which is almost never very "clear" when magnified that many times.
If you have ever watched TV
video of say, the space shuttle launches, you were indeed watching something going very fast, and a lot farther away than 7 miles; but, it was being filmed by a tracking camera which probably cost the taxpayers about 5 million dollars ! ( and was designed to do exactly that task, by a team of engineers who all get paid "big bucks".
Having had many years experience "fooling around" with telescopes and cameras, the closest thing I can remember that even comes close to tracking an airliner size object..............going real fast.......a number of very bright amateur astro-photographers have been able to track and film the space shuttles while in orbit, with only a 10 inch Meade ( or Celestron ) catadioptric telescope; (the trick in this being, a very sophisticated home made mounting (which was computer controlled), and some very sophisticated home-made software to tell the mounting where to "point" ; and you just could tell it was a space shuttle, ( which was a few hundred miles high)
Try this........next time you're in a big store that sells high end binoculars, ask to look at a pair of 10X binocs; while hand holding them to your eyes, try to read anything written on a sign in the store; you will be seeing what 10X magnifying the "shake" from hand holding does to your effort to see anything; better yet, if you ever happen to be where they shoot at targets with rifles, try hand-holding a 40X spotting scope ! You will quickly see why professional photographers spend big bucks on high-end tripods, to hold their "long lenses" !
Stupidity: Doing the same thing over and over again and expecting a different result; Albert Einstein