To go further I think it's not exactly open source but remote-first that made software development scalable.
If you grossly simplify it to its innermost core, making development scalable means that if you have 10 times more people you can have 10 times more features/bugfixes/speed improvements/... The only way to do that is to make sure that an additional developer doesn't depend on other developers to work, and that can only happen if everything is properly documented, the build instructions are up-to-date, the processes are clear, basically anyone can start from scratch and get up to speed without the help of anyone else.
That kind of organization traditionally doesn't happen in on-site companies, where newcomers are followed by senior people, they have to follow some introduction course to familiarize themselves with the processes, they have to ask many questions every day, they need to be synchronized with other persons which brings some inefficiency beacuse everyone works at their own pace, etc... This all disappears when everything is properly documentend and every contributor can work in the middle of the night if they wish. I think the Gitlab Handbook goes over this quite well and describes a framework to implement that kind of organization but the rules are retrospectively obvious for people already used to open-source (https://about.gitlab.com/company/culture/all-remote/guide/):
- write down everything
- discussion should happen asynchronously. Any synchronous discussion (by text or call) should be only very small points. Whatever the type, write down the conclusions of that discussions
- Everything is public (to the organization), including decisions taken, issues, processes