How all new & old WiFi products are competible with each other

how the WiFi specification can make it?
i would think that if there are many people develping WiFi there must be some products that are not compatible with each other, am i wrong..... is everything in WiFi products is perfect?
if yes how they did it, how they could specify the system so perfect?

the WiFi specifications are backward compatible and go thru exhaustive testing to be as close to perfect as possible.

Design implementations are another story. How operating Systems influence connection speeds is less than perfect. For example a laptop path to its home router may bounce off many walls before getting to destination. If these signals appear as out of phase "echos" at microwave lengths at the same amplitude, they cancel and that dead spot may only be a few mm every 10cm. Meanwhile the user is oblivious to this condition and thinks he/she is close enough to get a good signal. So the operating system driver tells the Wifi adapter to drop the connection speed and try again. THis may affect the path and dropout enough to be above the noise threshold and work. A better solution is notify user that the signal is marginal or "strong signal with strong reflection noise" and move to a different direction and hold the mobile device steady.

Education is key in the above example and a scholar may read about Rician fading but educating the user how to make Wifi compatible is not hard as it generally works universally when the signal is strong. When it is weak, then sources of interference including its own signal with walls and cordless telephones on the same channel are the most common issues.

if for insteance i followed the Wifi spec & did everything right can it assure i will pass the interoperability tests?
or these test are seperate benchmark during the new product aprove? i'm asking becuase i can't understand what can be the methods (regorozic and strict as they can be) methods that will assure that my e.g. new product will work with future products.... i mean what are they testing? what they could define so perfectly in the testing that make the products so compatible?
as i know RF is a very woodo realm and maybe some changes in new products desgin are unpredictable even by perfect testting....am i wrong?

There are many detailed test plans for Wifi you can find. But yes the spec covers all the requirements and the test specs are based on the design requirements.

**broken link removed** e.g general test plan.

IEEE 802.11 a/b/g etc. are generally considered to be the WiFi "specs". However, these specs are MAC layer specs primarily, they have very little detail about physical layer. Years ago it was found that companies could be compliant with IEEE 802.11 specs but still build devices that were not interoperable. The Wi-Fi alliance (separate organization from IEEE) tests to make sure different manufacturers products are actually compatible with each other.

https://www.wi-fi.org/

Great thanks, so now i will ask the big Question....
for insteance apple is making their new product that they reveal only after it develped
and e.g. they include WiFi module, so do they develp it by using only the iEEE spec? or they have some other documents?
if they develped almost without the RF layer (only by IEEE spec) there probobly might be interoperabilty issues
than how it is solved? will the WiFi alliance lose a lot of mony and not give to Apple the interoperability certification?