Do Japanese prefer white fish or red fish?

No preference or individual preference.

red meat( Maguro family ) are best eaten raw. When cooked, its more or less canned tuna.

Japanese generally don't eat shark even though its so good and tasty.

What do they prefer for sashimi?

Both.

Each type of fish has its own flavor. for large fish, different parts have different tastes. Probably the best example is Blue fin tuna(Maguro). Its most prized part is the fatty bottom area(Toro). The meat on their backbone is scoop with a spoon and and mixed with green onions and made into Tataki.

Besides the common fish at Sushi bars, many local fish make excellent sashimi. In SoCal, Rockcod and Cabison(Sculpin) make excellent white meat sashimi often eaten on the fishing boat.

Does the Japanese people cook some kinds of sashimis frozen?

Sashimi is eaten raw. It can be a part of dinner, but its usually food to go with drinking.

The one exception to the 'raw' form is Sashimi Ochzuke. Place several pieces of white meat sashimi on a bowl of rice. Pour hot tea over fish and rice, right up to the rim. Add a little soysouce and dig in!

Basashi (Horse sashimi) and Gyusashi(Beef sashimi) are eaten partially frozen. Its very good.

Locally in Japan, anything might become sashimi. I've heard of raw chicken, wild boar, even expensive mushrooms become sashimi.

Sakhas like horse sashimi and sashimis from fresh northern fish like muksun or chirh, both are served frozen and very thin sliced.

Are those fresh water fish or salt water fish?

Wow! Another group that eats horse sashimi. What part of Asia do the Sakhas live in?

Russia.

That's cool! I've forgotten that Russia has a sizable Asian population.

What do you guys think about the Japanese and its culture?

I don't know since I was born in USA, and had lived in russia for only 6 years till 2000 when the whole "democracy in russia" has gone belly up.

Most of USA Sakha don't even know their native language and have almost forgot the traditions. And I was surprised too when I was given the "oyohos" (frozen horse sashimi). And these way I think most of USA Sakha thinks about Japanese people as an ordinary USA citizen would. As for the mainland Sakha I think that they are very optimistic about economic cooperation and so on. And I was surprised to see a sizable herds of otaku which overwhelms those of USA too.

>Are those fresh water fish or salt water fish?
Only non red freshwater fish are used for it. Since it has a lot of fat, and no salt so it wouldn't be turned into chewing gum by freezing, and as much of fat it would have it would be the better. If you have so you could try yourself.

Freeze the fresh muksun completely, cut off the skin and fins, than begin to slice the fish into long thin stripes from tail to head with big and unbendable knife (non extra sharp would be ok as it would allow to control the thickness more precisely and wouldn't cut your hands). You may leave the lenght as it is or cut it to chopsticks pickable lenght. Eat with medium grain salt and black pepper.