Pre-installed plugins are in /app/code/venv/lib/python3.12/site-packages.
Marketplace/Custom/User plugins are going to be installed to the default upstream location /var/pretix/venv/lib/python3.12/site-packages.
source /var/pretix/venv/bin/activate
(venv)$ pip install <plugin name>
After that, the app can be restarted to rebuild plugins and apply db migrations.