I just keep it simple, such as W3C:

Definition and Usage

The alt attribute provides alternative information for an image if a user for some reason cannot view it (because of slow connection, an error in the src attribute, or if the user uses a screen reader).

Note: The alt attribute is required for the <img> element.

Note: For <input> elements, the alt attribute can only be used with <input type="image">.

Tip: To create a tooltip for an image, use the title attribute!

using a text-based browser such as lynx will answer all your alt text questions.

Does this mean that if I never alt - I'm making a mistake? Adds a photo title and description - but never alternative text. Should I fix it?

Should I add the same thing as the description or something completely different?

The alt="text" is what is read instead of an image so that people with vision problems can understand what they can't see. The Alt text should describe the image. It also allows reading software to describe what is not included in the text.

Google offers a How-to group of articles to help site creators learn the what and why of such details: https://web.dev/

Adds a photo title and description - but never alternative text.

What do you mean by “description”? That's not an html tag. In general, the alt is a description; that’s why it exists. Similarly, what do you mean by “title”? The <title> tag, or something else? Titles are rarely necessary with images, unless you’re adding optional extra information (looking at you, xkcd) or it’s text-as-image.

I have been known to use “[see caption]” * as the alt text, where an image already has a long descriptive caption. But that’s more likely if you’re reformatting existing material, such as an old book.

* Also, in rare and special cases, “clay pot” repeated hundreds of times. But generally this is Not Nice.