At the same time, some big projects can earn money by offering enterprise support [1], selling courses/certification [2] or by charging for repository packages. [3]

However, how do medium-sized players like Wireshark, GIMP or LibreOffice survive? I'm not sure that donations make a huge difference and it doesn't seem they have some support plans.

[1] Ubuntu: https://ubuntu.com/support

[2] RedHat: https://www.redhat.com/en/services/training-and-certification

[3] Conda package manager is free of charge but for using the Anaconda repository, enterprise users have to subscribe to a paid edition: https://www.anaconda.com/blog/anaconda-commercial-edition-faq