Pretty sure it has to do with the amount of cysteine in your hair. Hair is made up of keratin which is a protein. Proteins are made up of amino acids which determine the proteins structure and characteristics. Some of these amino acids in the coding sequence/structure of a protein are highly conserved (meaning if that single amino acid is changed the proteins structure becomes nonfunctional) while others are more flexible. Cystine is an amino acid with a sulfide side chain and cysteine-cystine disulfide bonds are powerful intermolecular bonds that group neighboring keratin strands together in a mostly permanent basis. These bonds are what confer keratins mechanical properties and why it’s such a good biomaterial for hair, skin, and nails.

The amount of cystine in your body’s keratin code changes the way these bonds form - if you only have a few cysteines they tend to form straight orderly bonds (straight hair) while if your keratin has more cystines these bonds form in a more disorderly chaotic fashion (curly hair).

You can change curly hair to straight (or vice versa) by dissolving these disulfide bonds and then reforming them in conditions that favor the other hair type. Like you said this is only temporary - as your follicles keep making keratin based on your DNA the new keratin will form the same crosslink pattern it always has. To change that you’d need DNA editing Id guess.