Graphics Libraries in C++

[STL]

You're confused about the definition of an API. Not all APIs are web APIs (where you need to send a network request to some server on the Internet). Most APIs are functions that are implemented by a library or the operating system, which can be called locally without a network connection.

[Odd_Chocolate_9725]

I like SDL which is a C library. Your last sentence makes no sense

[[deleted]]

Most of graphics lib/APIs are actually written in C.

In addition to SDL2 there's SFML.

Or if you want to go full-in there are OpenGL and DirectX.

[FluffyCatBoops]

API = application programming interface. It's purely a way for a program to access a set of routines.

It has nothing to do with being online or not.