How to use this if I am an SQL beginner? For example, I can see some relations for some set of tables but I want to see the code which resulted in the relations. I do not see any field for inputting some SQL.

This tool doesn't let you run SQL queries or create tables. SchemaSpy describes and visualizes an already-created database.

To work directly with SQL, I recommend https://www.beekeeperstudio.io/ or https://dbgate.org/

This is the most helpful DB visualization tool I've found. It really helps me figure out what queries are possible to find what I need.