I will try.
In general, I find most people attractive in their own way, so I have difficulty defining what is and is not 'beautiful' in humans.
People who fit the current trend of beautiful (and by this I mean what a culture finds beautiful during this century/decade) are more desirable in our society because beauty has power, but it doesn't mean they are the only people who are beautiful.
Good health, symmetry, homogeny or contrast (in hair, skin and eyes), vitality, etc, are all typically circumstances that add to a person's beauty. A person at their best in terms of health, mental wellness and life's ups and downs, is more beautiful than the same person at their worst.
We can argue that youth and signs of fertility are beautiful, but this is all intertwined in "good health". Personally, I find these arguments too basic and primitive.
I find potential beautiful: future potential, present potential, and past potential.
Personality, "heart", and intelligence play into beauty. People who are balanced, accomplished, kind, compassionate, talented, intelligent, and have grace, wisdom, etc, are more attractive. Less desirable traits can make people unattractive. For example, a cruel person is unattractive.
We can argue back and forth about the golden ratio in human beauty. Yes, OK, people who fit the ratio are traditionally more beautiful, but there is something fascinating about the people who don't fit, too.
In nature an animal that is healthy and well is more beautiful. We would certainly say a deer in top health is more beautiful than one with a wasting disease, no matter how beautiful the diseased one used to be. This may or may not work for plants. There is something beautiful about a tree that grew twisted and bent, or an old tree that is a bit broken somehow. Why? Because that tree stands apart from other trees, and because humans usually see trees as objects instead of beings. Objects that have a unique shape are more interesting and memorable than homogenous ones.
Outside of the quagmire that is defining human beauty, we can dive into the golden ratio, Fibonacci, and color theory.
Art that follows mathematics is subconsciously more attractive than art that does not. Humans find mathematics beautiful in music, architecture, design, and nature. Yes, even those of us who say we "hate math" subconsciously find math beautiful. However, keep in mind that math is a human language invented to describe nature.
Leonardo da Vinci was an expert at using mathematics in his art, and the Mona Lisa follows mathematic theory. Personally, I'm enthralled by Virgin of the Rocks, not Mona Lisa, but I know why everyone else likes the Mona Lisa. Three factors: Mastery (talent and experience), math, and sfumato. Sfumato is a technique that creates soft transition of colors, like a haze or smoke. It mimics how the human eyes sees (what is beyond the focal plain is softened), makes a more "believable" image and gives an atmospheric effect. Nobody did this better than he.
Caravaggio, in ...hahaha... contrast... is the "father if chiaroscuro". Chiaroscuro is a strong contrast between light and dark. Light is an important factor in beauty.
For color: Analogous, monochromatic, triad, complementary, compound, shades, tints, tones. There are many different color relationships, but settings that follow those relationships are considered more beautiful than those that do not. The biggest exception to these rules that I can currently think of is an "explosion" of a large assortment of colors, such as in a flower garden, confetti, or other collection of dozens of colors.
The color relationships that appeal to us change with age, too. Children prefer bright, adults prefer tones. Tones are deemed more 'sophisticated'.
An interesting factor I learned as a color printer: People with blue and green eyes printed the imagery slightly cooler and people with brown/golden eyes printed slightly warmer. (This was a problem for me because I printed cool and the production manager had brown eyes and preferred images to be slightly warmer. Even half a point warmer looked too warm to me, while she thought my images were too cool.) So, what we find attractive may vary slightly based on our anatomy, at least our eyes.
Women were also generally more successful color printers than men, and the industry of color printing was dominated by women. (Yes, there were talented men, too, but not as many.) In our lab, brown and hazel eyed women were the majority. Professional photographers, museums, galleries, universities, etc, from all over the world used our services and believed we were "the best", so how our production team "saw" beauty mattered.
Light is beautiful. How it shines, is filtered, reflects, creates contrast, haze, creates a glow... etc, etc, etc.
Sunset and sunrise are light and color theory. The sun dominates beauty because it is our life force.
Destruction, devastation, disease, cruelty are not beautiful. If we find beauty in these situations that beauty is hope and potential, something that survived and will thrive again.
Emotion. How an image or song makes us feel is beautiful, even if it makes us sad. Melancholy music is some of the most popular. Anything that helps us feel less alone, more enlightened, opens our minds, touches our emotions, helps us reach intellectual or spiritual depths... is beautiful. Love is beautiful. Compassion, kindness, perseverance, bravery, selflessness.... are all beautiful.
Skill and talent are beautiful. Hours and years of practice and study that show in the final presentation (an object, art, music, food) are beautiful.
Raw nature is beautiful. But, raw nature usually follows some of the relationships and features I've already mentioned.
Fashion is not necessarily about beauty. Style and beauty are different, though the two dance together.
Make-up is usually, but not always, about beauty. Makeup mimics health, creating contrast, and exaggerates features. What is truly sad about this is that make-up has become so synonymous with beauty that the natural face is no longer considered as beautiful. Sure, your girlfriend/wife is beautiful without make-up, but most people will say a woman they see daily who isn't wearing make-up isn't as beautiful as a model or actress. More often than not the model or actress is using makeup, even if it doesn't look it. In cases where she is specifically stating she is bare-faced, in most cases the lighting and camera angle are carefully chosen to be more flattering. So now we are back to the importance of light in beauty. Lighting can make or break a photo or film.
Next, ask an insect what is beautiful. It is something entirely different.