After installing RHEL 9.1, sudo yum update complained:

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There is no enabled repos in "/etc/yum.repos.d"

Every blog post I found swaps in CentOS repos instead. But RHEL is now free for individual developers, so the first step is to register a Red Hat account, get the (free) Red Hat Developer subscription approved, and then register:

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sudo subscription-manager register
sudo subscription-manager refresh

Along the way I hit

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System certificates corrupted. Please reregister

and refresh failed. Searching turned up that a file is missing from /etc/pki/product. Download Red_Hat_Product_Certificates.sh from access.redhat.com/labs/rhpc and run it; registration and refresh then succeed and yum works normally.