Why is all hair different? For example, why do some people have straight straight hair and some people’s have curly hair? Is it not just dead skin cells?

Different hair types are beneficial for different environments.

Kinky/curly hair is better at regulating hot temperature than straight hair. This may have given humans in warmer climates an advantage.

On the flipside, straight hair retains heat better, and is better suited to colder climates

Hair is ABSOLUTELY NOT dead skin cells. Hair and skin both use a lot of the protein keratin, but neither one makes the other. The hair outside your scalp is effectively dead, though.

It depends on the shape of the hair follicles (and genetics), which change change due to various factors. Round follicals will produce straight hair, flattened ones will produce different types of curly hair.
And yes, hair itself is dead (just like our nails) but the (healthy) follicle is “alive”.

Hair is made of keratin and keratin strands are made up of collagen which is a protein
When that strand is linear its because the individual amino acids are interacting with their amino acid before it
Sometimes tho(determined by genes) one amino acid becomes charged and interac5s with an amino acid three four positions behind causing the collagen strand to bend (this happens in thousands of strands ) ans this cause ppl to have curly or wavy hair

Comes down to genetics. Some people may code for a keratin protein that can form more disulfide bonds. These covalently link parts of the protein together, causing a kink in the protein. https://pubmed.ncbi.nlm.nih.gov/22683767/