A lot more planes are leased (rather than owned) than one may think. Even for a typical "standard" commercial passenger jet, a decent amount of "customization" occurs. Things like seats, cabin layout, etc. are all done irrespective of the lease on the actual jet. The leasing companies don't care what is done as far outfitting as long as they receive payment on the lease.

Jet leasing companies actually present an interesting investment for someone who wants some exposure to the airplane manufacturing, airline, or air shipping industries but without the volatility or liabilities of an actual airline or company like Boeing. They also tend to pay extremely high dividends.