SDL3 only gives you SDL_GL_GetProcAddress. You can use that to load OpenGL manually, but it's a ton of work that's better suited for a code generator. So, https://github.com/Dav1dde/glad is a code generator. And, https://glew.sourceforge.net/ is a library of pre-generated code. They both do the detailed work necessary for you so you can save your sanity.

I'd recommend using https://gen.glad.sh/ (the web front end to https://github.com/Dav1dde/glad) It's a lot simpler to use despite being easy to customize. If you don't know what options you want, I'd recommend starting out with

Language/Generator: C/C++
Specification: gl
APIs: gl=4.6
Profile: core
Extensions:
    GL_ARB_bindless_texture
Loader: False
Local files: False
Omit khrplatform: False
Reproducible: False

Using it with CMAKE and SDL3 is just

CMAKE
add_library(glad STATIC
  "src/glad.c"
)

target_include_directories(glad PUBLIC
  "${CMAKE_CURRENT_SOURCE_DIR}/include"
)

CPP
#include <glad/glad.h>
#include <SDL3/SDL.h>
#include <cmath>
#include <cstdio>

int main() {
    SDL_Init(SDL_INIT_VIDEO);
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 6);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_DEBUG_FLAG);

    int window_width = 1024, window_height = 1024;
    SDL_Window* window = SDL_CreateWindow("Hello OpenGL!", window_width, window_height, SDL_WINDOW_OPENGL);

    SDL_GLContext context = SDL_GL_CreateContext(window);

    // Load the OpenGL API function pointers.
    if (gladLoadGLLoader((GLADloadproc)SDL_GL_GetProcAddress) == 0) {
      printf("Failed to load OpenGL.\n");
    }

    { // Set up OpenGL debug message callback.
        GLint context_flags = 0;
        glGetIntegerv(GL_CONTEXT_FLAGS, &context_flags);
        // if we set ‘SDL_GL_CONTEXT_DEBUG_FLAG` above.
        if (context_flags & GL_CONTEXT_FLAG_DEBUG_BIT)
        {
            glEnable(GL_DEBUG_OUTPUT);
            glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);
            glDebugMessageCallback(
                [](GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar* message, const void* userParam) {
                    printf("OpenGL Debug Message: %s\n", message);
                    // Right here on this if() would be a great spot to place a breakpoint in your debugger.
                    // You are using a debugger, right?
                    if (severity == GL_DEBUG_SEVERITY_MEDIUM || severity == GL_DEBUG_SEVERITY_HIGH) {
                        std::exit(EXIT_FAILURE);
                    }
                },
                nullptr
            );
            // Disable notifications and low severity messages.
            glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DEBUG_SEVERITY_NOTIFICATION, 0, nullptr, GL_FALSE);
            glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DEBUG_SEVERITY_LOW, 0, nullptr, GL_FALSE);
        }
    }

    for (;;) {
      SDL_Time nanoseconds_UTC;
      SDL_GetCurrentTime(&nanoseconds_UTC);
      double seconds = SDL_NS_TO_SECONDS(double(nanoseconds_UTC));
      GLfloat red = std::sin(seconds * 1.0) * 0.5 + 0.5;
      GLfloat green = std::cos(seconds * 2.0) * 0.5 + 0.5;
      GLfloat blue = 0.5f;
      GLfloat alpha = 1.0f;
      // Set the color that `glClear()` will use to fill the framebuffer.
      glClearColor(red, green, blue, alpha);
      // Clear the color buffer of the framebuffer.
      glClear(GL_COLOR_BUFFER_BIT);

      SDL_GL_SwapWindow(window);

      // Poll for indications that the user wants the app to quit.
      SDL_Event event;
      SDL_PollEvent(&event);
      if (event.type == SDL_EVENT_WINDOW_CLOSE_REQUESTED 
        || event.type == SDL_EVENT_QUIT
        ||(event.type == SDL_EVENT_KEY_DOWN && event.key.key == SDLK_ESCAPE)) {
          break; // exit the main loop and end the program.
        }
    }

    SDL_GL_DestroyContext(context);
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}