I wondered how open source works. In the case of @Tuta_Official for instance, couldn’t they just copy the import mailbox feature of @Proton_Team ?
Or if they don’t want to do it the same way, to inspire themselves from it (or another company) ?
Does companies sometimes take “inspiration” of the work of other open-source projects ?
A bit like the work @SkewedZeppelin is doing with DivestOS, porting features from GrapheneOS.
This is a question for FOSS developers who can explain better. I think it has to do with the type of incorporation of the FOSS code from elsewhere and the dependencies it needs which may not be fully vetted by another FOSS developer.
It’s essentially a matter of providing proper credit if you have reused some open source code into your project according to the license it was released with. Yes, unless you have an IP over a feature/project, companies can take “inspiration” of the work of open or closed source projects.
I’d recommend reading about how Linus Torvalds gave the world Linux. 
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