From mainframe to mobile open source operating systems have a deep history
Once upon a time operating systems were owned and managed by single companies.
IBM ran its System 370 mainframes on on MVS, VSE, or TPF. That in the 1960s and 70s. Things have moved on apace since, not least with IBM.
In the ICL (International Computers Limited, now part of Fujitsu) developed VME (Virtual Machine Environment) its mainframe operating system to drive ICL’s then new 2900 Series mainframes, the operating system is now known as OpenVME and has Unix elements.
So the story of open source operating systems can be traced to the mainframe era.
Also in the intervening decades the advent of the mini computer saw firms such as HP, Digital Equipment Company, Sun Microsystems all develop proprietary hardware platforms based on their own chip architectures and monolithic operating systems.
Digital Equipment Company (DEC) had VMS, Sun had Solaris and HP had HP-UX. Not to be outdone IBM introduced the AS/400 which ran on the OS/400 operating system.