hello! 520 actually came to be from online platforms who use it as a short form/slang for I Love You, kind of like how we text "ily" instead of the whole phrase? so bc 520 sounds like ily, it's widely used as a substitution instead of 我爱你 :)
numbers actually mean a big deal in chinese, & certain numbers, over time, become associated with different things because they sound similar to those things. a common example would be [i love you] 1314, which translates to [i love you] one life, one lifetime (forever). in this context, 1314 sounds like the words "一生一世", something like "one life, one lifetime" in chinese. another example would be the number 4, which sound like death (死) in chinese, hence it's considered an unlucky number.
hope it helps explain some things!
ヾ(❀╹◡╹)ノ~
520 = 我爱你, which translates to I LOVE YOU in english. so sweet!! ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄