Getting a phone number & insurance in Japan

I'm moving to Japan for College this year, and my biggest question is how I should go about getting a phone number, insurance, and healthcare.

I want to buy a completely new phone (a flip phone) in Japan, I am wondering where is the best place to buy a sim card from and receive a Japanese phone number.

For insurance, I will be 18 and still on my parents health insurance in the US. Is that health insurance usable in Japan for things like yearly checkups or getting medical prescriptions for things like antibiotics or flu shots?

I have done a lot of research, but without my web location being in Japan makes it difficult, additionally most resources are for foreigners going on vacation to Japan or trying to get a phone number online for accessing tickets etc. Even any good resources for researching more on these topics would be helpful.

Thank you!