It is not possible to create Flow and make it available for whole tenant to be used in any library/list in SharePoint site.
As there is possible to pick Flow from public gallery. It will be nice to have something similar on tenant level as mentioned in previous post here Reusing flows post.
It will be nice to create something that will be called Organization gallery so the Flow will be findable here for any user shared with.
It's now possible to reuse copies of flows inside of your organization: https://flow.microsoft.com/blog/send-a-flow-as-a-copy/
Any news on this?