How is komi calculated and why did i get the handicap advantage I was higher rated
These all depends of the game settings before proposing or accepting the game (custom with different options including rules, automatic…)
So no universal answer. You can check the FAQ on the main site for details
From the info of your game, the komi was 0.5 with Japanese rules which (I can guess that only) is a one stone handicap game. Why you are black and not white, that may come from custom settings made by you or your opponent.
Hi and welcome to the forums.
As far as I can tell this was an quickmatch game. There is currently a bug where handicap is sometimes assigned the wrong way around.
If you like auto-handicap; I recommend posting your own custom games and make sure that colour is set to automatic as well: Finding (starting) a game · online-go/online-go.com Wiki · GitHub
For all kind of info about go and OGS see the community generated info base.