New year starting on January 1st is indeed very arbitrary. New year has started on many different dates through different cultures and history.
If I remember correctly [citation needed], new year used to be March 1, which is why November and December reference novem (9) and decem (10)
Another arbitrary thing is the base-10 number system. We have nine seperate number symbols but do something different at 10. Why not twelve number symbols? Or Eight? Very arbitrary.
As well as dividing big numbers by thousands. Some cultures divide by myriads, so 1 million (1,000,000) to them would be 100 myriad (100,0000)
Everything is super duper arbitrary.