For example I'm trying to get set gear... it shows a video on YouTube of how to get there and I'm there but there's no portal ? Why not ? I can't find anywhere how to open it or why its not there..

You need 6 set bonus of a set to access it's set dungeon.

So ur saying the only thing I can do is keep killing random shit until I get set pieces ? .. is there not a easier way ?

You also get a full set really early while doing the season journey. Only one per season.
Other than that, just kill stuff.

Next question

Where does the dungeon spawn after getting a full set

asia hardcore?

Found this out threw Google,

Also set dungeon is just basically nothing more then an achievement....weak

Achievement and challenge if you want to call it that. More like a puzzle. A simple puzzle.

Wrong.

Set dungeons are required for the seasonal quests. Completing the seasonal quests give tangible rewards, such as extra stash tabs.

They are hiding