Leaving this for future seekers:

By default, when installing Oracle XE the ORACLE_HOME environment variable was set to /opt/oracle/product/21c/dbhomeXE

If you look in /opt/oracle/product/21c/dbhomeXE/hs/admin, you'll find all the sample files you'd expect for configuring the gateway.

I don't know when Oracle XE installs started doing this, but there's also another home directory that was created: /opt/oracle/homes/OraDBHome21cXE

If you look in /opt/oracle/homes/OraDBHome21cXE/hs/admin, this directory is empty.

I was putting my configuration files in what in /opt/oracle/product/21c/dbhomeXE/hs/admin. When I copied my configuration files to /opt/oracle/homes/OraDBHome21cXE/hs/admin, I started to get error messages that I can work on fixing.