If with me in the WLAN theoretically 10 humans at the same time netflix streaming the Internet goes slower that is clear, but how can one measure that or on what does this affect? Ping? Upload? Download?
Of course this affects everything as it looks like with your current values (download, upload, ping) you can e.g. Learn about https://www.speedtest.net/...dtest.net/.
The entire bandwidth is shared among all users. The more requests from these users come (page views, file retrieval, etc.), the more the latencies (ping) go to their knees.
There's a single reliable way to measure speed: WLAN off, only one device per LAN to the router and then through different server speed tests / download tests to make sure that the server is not slower than the port.
Well, first you should learn to distinguish between LAN and WAN.
This is the basis for understanding everything else.
Also, the topic "What is radio and how does it work?" with WLAN the basis.
After that, once you have internalized these basics, you can go to the step of analyzing data throughput, and so on. Without the basic knowledge you will not really be able to understand and derive something.
Maybe you start http://www.olivergast.de/blog/2011/04/16/netzwerkguide_1/. But that's really just the beginning.
The others' speedtest links are nice - but useless if you can't judge the result. For that you need basic knowledge.
