I was wonder if there were any way to make my own presets containing my own custom made brushes and to duplicate them if needed.

I sort of want krita to be as similar as paint tool sai but I have no clue which brush lets me blur anything nor do I know how to adjust and move the opacity and size to the right or left side.

You can lear a lot about Krita there: https://docs.krita.org/Main_Page
Also some video tutorials may help.
How to make brush packs (bundles): https://www.youtube.com/watch?v=vvB2pW5Q_hM
How to edit your brush presets: https://www.youtube.com/watch?v=qHC3h2aTse8 (this is a livestream, but is an in depth explanation).