The best 16-18 year olds don’t play high school hockey. In the US, before college hockey, guys with NHL prospects usually play junior (ages 15-20), and tier 1 is the USHL. The very best play for the US National Team Development Program in Michigan. Their U18 team plays in USHL as well as represents the US in the world U18 championships in Europe every year.

Depending on when a player’s birthday is they may be eligible for the draft before playing college hockey or after their freshman year. So the top American draft picks typically come from the USNTDP or NCAA (Michigan, BU, BC, Denver, Minnesota, Wisconsin, North Dakota, etc) and some from the USHL, though usually not at the top of the first round.

For Canadians, they typically come from major junior (basically the equivalent of tier 1) which is the Canadian Hockey League, which refers to 3 different regional leagues of about 20 teams each. Occasionally a top player will stick with Junior A (tier 2) to maintain NCAA eligibility (Cale Makar, for example.

Europeans usually stay home and play in their country’s top junior league or even maybe the top senior men’s pro league. A handful will come over and play a year in the CHL, usually to hasten the transition (and showcase to scouts) speaking English, North American culture, and especially North American size rinks/playing style. This may happen before they are drafted (Jakub Voracek) or after (Nikita Kucherov).

This is just generally what happens. There are a number of high profile examples of top Canadian prospects play NCAA hockey (Cale Makar, Jonathan Toews, Owen Power, Adam Fantilli) and of top American prospects playing Canadian junior hockey (Patrick Kane, Seth Jones, Matthew Tkachuk). There’s even been North Americans going to play a season of pro hockey in Europe rather than NCAA or CHL before they are drafted (Auston Matthews).