Many of my goals completion comes from Google / Organic according the Source / Medium report. But When I select LandingPage as Secondary Dimension 99% of them comes from "/", which I understand is the homepage. I know I can get all the queries my homepage is ranking for by Search Console, but how can I get what queries are the ones bringing me conversions?
Re: Orgnic Traffic
Actually you can't - keywords aren't provided here anymore, see for example  for discussion. Usual workaround is to use organic landing page instead.
In your case, look for the search terms that contribute most clicks in Google search console.
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