How to add own html and php code

I also work just locally on a OXID eShop for me.
I wanted to try to add to the "select an adress" process (Step 2) see Picture a Select menu to add the content from the MySQL database select the contents with php and mysql and after the order or within the order process, insert this information into the databse.
Because I am completly a newbie in OXID eShop and the programming language used for developing OXID, i need imidiatly your help.
I would be very grateful, if you could help me and look forward to your answer.

Note: I am using the Community Edition Version 4.9.0 :)

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