Drupal 8 : How to write custom php functions and call the function from a custom page?

I am new to Drupal web development. I have already used Wordpress platform by building a custom plugin to write custom php functions, and calling the function from a custom page was easy.

• How can I achieve similar with Drupal 8?
• Do I need to create a custom module for writing the custom php functions?
• How do I call a function from a custom page?

Please help.