As a technical intern, is there any legal way to stay in Japan besides marrying a Japanese and studying abroad? For example, find a Japanese as a guarantor

First of all, there is no intern visa. Are you a technical visa, a study visa or a cultural exchange visa? All three visas may have interns staying in Japan.

Secondly, are you two married? If you are not married, you can't rely on any visa. Married people who have been married for more than 6 months can apply for a visit visa or a family stay visa for a period of 3 months to 1 year and 6 months to 2 years respectively. Need economic guarantor and proof of funds.

As far as you are concerned, I guess you are not married, are you? If she wants to go to Japan, she can only apply for a tourist visa. This has nothing to do with your visa. See you later. This is the only way.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.