But is it about skills? Is it about early learning? Maybe it is more about having a fire to learn, a value system that appreciates education, and a healthy curiosity?
Why does Finland do so well considering their schooling starts at 7?
My guess is that it's a complex mixture of things. But you certainly don't have a level playing field when the children start school: in the UK, we observe a large (more than 1 year: I've seen 2 years claimed) developmental difference by the age of 5 between children in the highest and lowest socio-economic status families. I think it's learning in the broadest sense: learning to talk, listen, ask questions, look at books, express needs, share, ...
Some of the things Finland does well are: nine months' paid maternity leave (well, the first few months are specifically maternity leave, the rest can be split as the parents like); free full-time high-quality daycare and early education from the age of 8 months; teaching as a high status profession (very competitive to enter).
(That said, the countries that do well in international standardised testing are diverse; Finland's model clearly isn't the only one.)