Self Hosted User Magement Tools

You might want to setup KanIDM. It speaks OAuth/OIDC/LDAP etc.

Therefore you can configure on your applications to use authentication via KanIDM and then you can simply administer your users in one place without having to worry.

There are many other tools but this one is by far the most modern, lightweight and powerful that I know.

Any database or spreadsheet-type app? Grist comes to mind.

It really depends on your needs, with what you're describing a simple spreadsheet would be all you need.

But other companies need to integrate with certain systems that have authentication where you want to allow/disallow access. These are jobs for LDAP, SAML, OIDC providers. I personally use Authentik but there's Authelia and Keycloak as some examples.

But by no means are those as simple as checking a spreadsheet. But you wouldn't need to check a spreadsheet because it's integrated in your login page

Excel spreadsheet?

Active Directory