MySQL JDBC integration into App from tutorial
You're not following Java naming conventions. Package names should be all lower case.

The JDBC URL is hard coded. This should be provided as a parameter. Or better yet, provide the connection with dependency injection so you can unit test it with mocks.

Worst "offense" is hiding the exceptions from the caller. Instead of printing the stack trace (which is only visible to the end user) you should wrap it in a domain specific exception. This way the application using your library can take proper actions.

When saving the number you list all columns as parameters in the SQL. Including the automatically generated index, which you insert as zero. That was confusing. You should list only the columns that you provide as "INSERT INTO numbers_tabl (number, min, max) ..." and let the database handle the columns that can be generated automatically (index and creation date).

Instead of returning a boolean from saveNumber you could return the RandomNumber object updated with it's new database identifier and creation date.

The names in the database table don't describe their purposes. Table us named "numbers_tabl" but it contains random numbers. The name should tell the reader what the table contains. Also the _tabl suffix is probably a bit pointless. The "created" column name suggests that the value is a boolean. It should be "creation_time". I don't remember MySQL data types from memory but it probably should be a timestamp, not just date.

The method choice comes from the tutorial, right? It's a bit weird to load the numbers into memory and access them from there. After saving a number you have to call loadNumbers before you get the latest number back from getNumbers. The getNumbers should load the numbers from the database. If you want to cache the numbers in memory then the saveNumbers should invalidate the cache, but that's probably a subject for a later tutorial. :) Also there are libraries for that (Ehcache for example).

The standard library for this purpose is JPA (Java Persistence API or Jakarta Persistence). It makes boilerplate SQL so much easier.