The terms can be a bit confusing at times. Here is an explanation, sorry if it is lengthy...
As a developer of workflows/apps/macros (i'll refer to as just workflows from now on) in a gallery, you are an artisan. An artisan can save workflows to the gallery and when they do that, they go directly into their private studio. The workflows in that private studio can only be seen, run and modified by other members of that studio. Some companies have multiple people in a studio for collaboration, other companies have one person per studio. this is up to the admin to determine how they setup studios.
When an artisan wants to share workflows with others (who are not in the same studio), they have two ways to do this. One is by placing it into the "Company's Gallery" and the other is by putting it into a Collection.
By placing it in to the Company'g Gallery, it is made available to anyone who has access to the gallery. It will show up on the home page and can be accessed from there. There is no permissioning behind it beyond whomever has access to the gallery. Those public workflows can be placed into Districts which are pretty much like folders for organizing workflows. This is done through tagging of the workflows. So any workflows that are in a District, are publicly available workflows but Districts can help with organization when you have a lot of workflows in your gallery.
Collections are created by artisans in studios who want to share workflows with others but want to selectively chose who should have access to them. they can share multiple workflows in a collection and have multiple users accessing them but the folks accessing are selectively permissioned to do so.
Hope that explains it clearly.
I think this training may also be helpful - https://community.alteryx.com/t5/Live-Training/Live-Training-Alteryx-Gallery-and-Best-Practices/m-p/...