OK, so Im stupid - how do I copy a file?

None of my usual Linux CLI seems to work w/in HA. I have certs from a parallel web server that I want to use w/ HA (letsencrypt is already running on the other web server), but I cant figure out how to copy them from a USB stick to the /ssl folder for HA to use. I cant seem to get out of the >ha CLI to the operating system to copy files or even list folder structure. I’ve installed the nginx proxy but obviously can’t configure it with my certs.

What am I missing here?

Maybe use the Samba Share app so you can just drag and drop.

The folder is ssh on HA, but make sure you are in the right place. You are dealing with containers and things are not always what they seem.
Like they said, use the Apps (formerly called Add-ons) and that keeps you within the OS. Yes HAOS is Linux, but not bare Linux. You are meant to operate within the structure and that means no obvious access to root.

Are you using this app for SSH? GitHub - hassio-addons/app-ssh: Advanced SSH & Web Terminal - Home Assistant Community Apps · GitHub

On mine I can get to the /ssl folder no problem.

root@HAOS-M710q - 17:10 /root # ls /ssl

fullchain.pem  privkey.pem

You CAN enable access to the bare system with developer SSH, but I’ve found Advanced SSH to be friendlier as it has apk built into it, and the only thing that doesn’t work so far is smartctl.