let's say they live in both OS and Router. what will happen if we try to use a modern technology like Wifi with an older version OS?like windows 95? Doesn't 95 have the data link protocls needed to communicate correctly with Wifi or something?
The OSI model is conceptual. In the example you provided the Win 95 system would require a driver to support Wifi.
Each layer depends on te lower layer to get to the wire. For example if you were to develop an application, would would code into your app how to place the 0's and 1's on the network media. You application make calls into the lower layer (other programs) so on and so on. The reverse happens on the target machine on the way up the OSI until it reaches the application layer. For example, wifi NIC vendors do not integrate code into their firmware for a specific target app.