Theory of multitexturing with texture objects and samplers

Question

I could not find any good theory articles on how to encode multitexturing only with texture objects or with texture objects plus samplers. I just don't know how to control the glActiveTexture function and what it exactly does.

glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0 + 0); // Number between 0 and GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, img.getSize().x, img.getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, img.getPixelsPtr()); // Not in sampler
glGenerateMipmap(GL_TEXTURE_2D); // Not in sampler

/* Values associated with the texture and not with sampler (sampler has priority over texture).
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);*/

glGenSamplers(1, &textureSampler);
glBindSampler(0, textureSampler);
glSamplerParameteri(textureSampler, GL_TEXTURE_WRAP_S, GL_REPEAT);
glSamplerParameteri(textureSampler, GL_TEXTURE_WRAP_T, GL_REPEAT);
glSamplerParameteri(textureSampler, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glSamplerParameteri(textureSampler, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);

glUniform1i(glGetUniformLocation(colorShader->program, "textureSampler"), 0); // 0 pour GL_TEXTURE0

Am I a little confused if multitexturing is due to the presence of several samples in fragment code associated with several textures, or if it has only one sampler with several textures?

Answer 1

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glBindTexture(GL_TEXTURE_2D, texId1);
glBindTexture(GL_TEXTURE_3D, texId2);

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glActiveTexture(GL_TEXTURE3);
glBindTexture(GL_TEXTURE_2D, texId);

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GLint loc = glGetUniformLocation(prog, "MyFirstTexture");
glUniform1i(loc, 3);

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glActiveTexture(GL_TEXTURE3);
glBindTexture(GL_TEXTURE_2D, texId);

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glUseProgram(prog);

GLint loc = glGetUniformLocation(prog, "MyFirstTexture");
glUniform1i(loc, 0);
loc = glGetUniformLocation(prog, "MySecondTexture");
glUniform1i(loc, 1);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texId0);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, texId1);

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Answer 2

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