Yes, Apache Ambari "is a completely open source management platform for provisioning, managing, monitoring and securing Apache Hadoop clusters", licensed under the open source Apache License. My understanding is that any source code contributions that Cloudera has made to Ambari are always contributed back to Apache and can therefore be obtained from Apache's source code repository if they can't be retrieved from Cloudera's. Some members of the Cloudera Community have reported success building or upgrading Ambari themselves from source.
When you write that you "used to be able to download freely from public repo but not now", that is because in the early months of 2021 Cloudera modified its download policies and the binaries are now only available in a private repository. Please see the announcement here: Transition to private repositories for CDH, HDP and HDF
However this change in how Cloudera makes binaries available doesn't change the fact that Ambari is an open source project licensed under the the open source Apache License. According to the Frequently Asked Questions list,
Is source code behind the paywall? No. Currently, only the binaries are secured behind the paywall.
Bill Brooks, Community Moderator Was your question answered? Make sure to mark the answer as the accepted solution. If you find a reply useful, say thanks by clicking on the thumbs up button.