For the homeassiant container, I do it with an automation, to run apk when HA starts
for work it is easier to install in vscode and not think about addons
I mean that wasn’t really the question though. Also VSCode is an addon too? Unless you’re talking about your non-HAOS system again. Which doesn’t really apply since this thread is in #home-assistant-os . For HAOS users its all addons. VSCode editing? Addon. SSH+Nano? Addon. Edit text files in the GUI? Addon.
I use a combination personally. Most of the time I’m using VSCode. But sometimes I want to tweak something on my phone. In that case I ssh in and use vim.
I just use notepad++ , and access files trough smb 
Anyway, the APK command is indeed usefull, to add additional packages like cantools/sngrep to trace sip traffic from asterisk addon … :+)
And indeed only works in the add-ons, not in HassOS itself there is no package manager there
Thank you @SVG62
To explain his screenshot:
HA → VS Studio Code Add-on (not via ssh) → Hamburger Menu → Terminal → New Terminal
apt update | apt install nano | nano /config/.storage/core.config_entries
Works for me! I was able to use this method to find what I needed.