How is a photo indexed without alt=""?

Question

Search Engine Optimization

Entered "boobs" into Google, got the issue, go to the source.

There I look for the keyword "boobs" in the tag, then there is not even alt in img.
How so?

Here is the site
https://www.drive2.ru/b/288230376152331632/

Answer 1

According to the general semantics of the text, and according to the text next to the picture

Answer 2

By surrounding text / headings.
Or on other sites where there is the same picture, but with alt.

Answer 3

By semantics - the nearest h1-h6 or text element on the same level with this picture. For example, in the bootstrap carousel, if the image does not have alt, then it is indexed by the text contained in the carousel-caption element (google the carousel markup if you don’t understand). If there are no headings at the next level above or below, then the same picture is searched in other resources, if it is of the same resolution or with the same file name, then it is indexed, if not, then the nearest headings are searched further down the house