In the Windows version of QGIS 3+, there is a relatively simple procedure to install more Python packages that will then be accessible from the QGIS Python console. It all starts with the OSGEO4W shell that you start as admin, then py3_env then pip installs. You can run into some trouble with conflicts in the dependencies, but you can easily add some potent packages like scikit-learn and -image, seaborn,pandas, etc.. without breaking QGIS
What's the equivalent procedure for Ubuntu?
More specifically, a Ubuntu 20.04 machine with an existing install of Anaconda and several conda environments.