Brave is definitely open source [1] (under the MPL)? The source for Safari is closed, but their rendering engine is completely open source [2].

[1]: https://github.com/brave/brave-core

[2]: https://github.com/WebKit/WebKit

Brave appears to be opensource: https://github.com/brave/brave-browser

While Safari is not open source, WebKit is[0].

[0] https://github.com/WebKit/WebKit