This thevenin theorem is really giving me a headache, and I've really searched many sources in the internet but they confused me more and more. Take this as an example:

The guy in the above link says:

The 6Ω resistor is short circuited and the 5Ω one is open. Therefore, their currents are zero and RTh=2Ω .

But see this link for example and especially this picture:

According to the above the $R_1$ and $R_3$ resistors should be shorted and the only one that should count is $R_2$.

And see this link also

the three links solve the thevenin theorem in different ways and especially in finding $R_{th}$ can somebody explain this to me in a simple way?

This thevenin theorem is really giving me a headache, and I've really searched many sources in the internet but they confused me more and more. Take this as an example:

The guy in the above link says:

The 6Ω resistor is short circuited and the 5Ω one is open. Therefore, their currents are zero and RTh=2Ω .

But see this link for example and especially this picture:

According to the above the $R_1$ and $R_3$ resistors should be shorted and the only one that should count is $R_2$.

And see this link also.

The three links solve the Thevenin theorem in different ways and especially in finding $R_{th}$ can somebody explain this to me in a simple way?