What is it?
And... it's free and open source: OpenUI5 is licensed under the Apache License, Version 2.0 - see LICENSE.txt. It also contains third-party open source modules. See the list and respective licenses in THIRDPARTY.txt. Third-party module license information is also available in machine-readable format in the .reuse/dep5 file.
Go to the download page and get the complete UI5 runtime and the UI5 SDK containing the documentation and many example apps as well as the complete runtime.
Or use UI5 Tooling, the recommended tool for working with OpenUI5! It allows you to manage your framework dependencies and to build optimized packages for all applications, components and libraries. Get started here.
Try the Hello World, read the Developer Guide and refer to the API Reference. Use App Templates as a foundation for your developments (available in SAP Business Application Studio or here on GitHub). Check out the SAP Community Forum for 20,000+ questions and answers and stackoverflow (use the tag "sapui5") to discuss code-related problems and questions. Or join the OpenUI5 Slack.
You can get the sources and build UI5 on your own, please check the documentation for the UI5 development setup. You might then want to understand how control libraries are structured and controls are developed. Maybe there's a bug you could analyze and fix?