They don't. Each country uses a different letter for their aircraft registration. All aircraft owned and registered in the US uses the "N" prefix.
Canada aircraft are registered with a "G".
Nothing. Social Security Numbers (SSN), Preparer Tax Identification Numbers (PTIN), Employer Identification Numbers (EIN), Individual Tax Identification Numbers (ITIN), etc... are all free, you just have to meet the requirements for each.
All kind of vehicles needs a name sometimes followed by a number just to identify it. May be such numbers would be associated to some feature of the vehicle, but the main reason is only for purposes of identification. In your question is lacking a name, Airbus.
Yes
Every military or civilian aircraft in the world has civil or military recognition or registration numbers. All civil aircraft numbers are led by a country code, for instance 'C ' for Canada, 'G' for UK, 'N' for USA and so on.
No since 2009 there are also ISBN13 numbers that start with 979.
A car VIN is a 'Vehicle Identification Number' and it usually contains 17 numbers, all cars have them and this prevents car theft in many cases. It is the identification for a SPECIFIC automobile
Military aircaft display national markings in order to be identified by other aircraft as enemy or friendly. Modern US Air Force jets use markings that lack color and are shades of the camoflage. Civilian aircraft are only required to carry a Registration identification number. This registration ID includes Letters and Numbers and each country has a unique prefix Letter(s). For example, all aircraft registered within USA begin with "N" and this registration is commonly referred to as the "N-number". However, I do not think a civilian aircraft is required to display the national flag.
same place as all cars. Dash, Engine exc.
Rational numbers, real numbers and complex numbers to start with. Also the set of all numbers.
They don't all start with 9. Mine doesn't.
Numbers are used primarily as an aid to identification. An aid to identification is not required in a game of tennis as there is only ever a maximum of 2 players per side. The identity of these players is easily known at all times during the match by the officials and so numbers are not required.
It is entirely up to the manufacturer of the aircraft. Usually it is an imaginary point forward of the nose of the aircraft and all points aft of that datum have a positive value. This makes weight and balance calculations easier as there are no negative numbers.