Some remarks:

1. Indeed, using open-source software does not mean one needs to be exclusive about it.
   For example, the Debian distribution does allow, and provides means to install, closed or even non-free software - the firmware for the wifi or graphic cards included. Same for Fedora. In both cases, the user must explicitely choose to do so but it’s only a matter of enabling the appropriate software repository.

2. Apple didn’t take over webkit as you describe. Webkitgtk is still being developed by Collabora and benefits from safari development.
   I would, however, be more reserved about what Google did to webkit a.k.a. blink - which has been made very hard to use in other projects. Electron devs surely won’t deny that.

3. Companies

• pay for a service to simplify setting up their service, preferably a service built upon open-source
• pay developers to build software, most of the time with several open-source components
• use open-source software as normal people do