Those 5 icon sets, Bangalore and Freemoji sets are all licensed under the Creative Commons - CC BY 4.0 which lets you use them as you want.

https://creativecommons.org/licenses/by/4.0/

Other premium icon sets aren't open-source, so using them would require a backlink or a purchase.

What does Open Source even mean in this context? To me this would mean getting the source files for the icons.

Creative Commons licenses aren't open source licences. Your phrasing is misleading. They are however copyleft as they allow for redistribution and modification and this is the word you should be using.

CC BY is not a copyleft license. Copyleft is explicitly about the requirement to use the same license for modifications, which is present in CC BY-SA, but not in CC BY.

CC BY is recognized by the FSF as a free license and by Debian as DFSG-free. While it's not listed by the OSI as open-source, I think that's mostly because it's not a code license. So I don't think that calling it a open source license is misleading.

Of course, for open source icons I would actually expect the source files to be open...

Where is the source for the icons and what format is it in?

Would a figma file and a github file help? https://www.figma.com/community/file/1063138616574654762 https://github.com/webalys-hq/streamline-vectors/blob/main/R...

So it is open source but has a dependency on the proprietary Figma tool for editing the source?