You can export the app artifact and share it. The artifact is human readable and in a JSX format (called Toolscript). In this way it can be meaningfully interpreted, improved and customized by others in an open source manner.
As for running the software the platform has a proprietary license obviously but you can host your own Retool backend from which to serve the apps to your end users (check the docker repo). This is an alternative to the SaaS/cloud based app hosting at https://retool.com/.
There's a free tier to edit and run the apps but professional usage will have a license fee. This is not unlike making an app with VB which required buying VB and having end users buy and run Windows.