CentOS Stream is a continuously integrated 'midstream' between Fedora and Red Hat. Red Hat is developed from it to provide subscribers with a stable distribution, but there are still parts that take place in private even though they've introduced a midstream that is much more helpful for accepting contributions.

OpenSUSE Leap and SLE always stick to the same schedule and share the same source code. Because they are developed together, the process for stabilizing it, putting out security errata and metadata, and ensuring compatibility -- is all in the public eye (aside from sensitive matters). The Factory and Build Service is where all the branched versions of packages are kept, and they have a policy of aggressively upstreaming.

SLE does have a very close equivalent to EPEL called Package Hub, which supplements the enterprise SLE with community-provided packages. (https://www.suse.com/c/how-suse-builds-its-enterprise-linux-... )