The following sections are introductions to the basic principles of the language of Mathics3. A few examples and functions are presented. Only their most common usages are listed; for a full description of a Symbol's possible arguments, options, etc., see its entry in the Reference of Built-in Symbols.
However, if you google for “Mathematica Tutorials” you will find easily dozens of other tutorials that are applicable. For example, see An Elementary Introduction to the Wolfram Language. In the docker image that we supply, you can load “workspaces” containing the examples described in the chapters of this introduction.
Be warned though that Mathics3 does not yet offer the full range and features and capabilities of Mathematica®.