Its basically part of the application/website which you cannot see as its mostly to do with server side processing and storage of data.
Some of the common server side languages and frameworks which is used are php, java, python, node.js, C# using .Net Core, laravel, slim etc.
These are basically used to write apis to process the data and interact with database such as Mysql, mongodb, Ms sql server, PostgreSQL, Oracle etc.
Backend programming languages & Frameworks
Backend programming help to build code that allows a database and an application to communicate with one another.
Php, python, .NET core using C# , java etc are most prominent languages used to write backend code. The code is processed at the server side and rendered at client side.
There are several php based framworks which are popular such as laravel, slim, codeigniter etc. There are several frameworks for other server side programming languages as well. We have Django and Flask for python.
With the advent of front-end frameworks such as angular, react, vue there is a demand for backend api frameworks (esp rest apis) and hence Node.js has become more prominent now. Its very fast in terms of speed.
Please do not hesitate to share your idea
Just discuss your requirements to take it further and make it a reality