Questions about doing an internship in Japan

Hi, so I may have the opportunity to do an internship in Japan but it will be financed by an organisation outside of the country (not japanese)

In such case, how does it work visa wise? Is it a work visa? About the proof of funding, as long as I have a paper that shows the quantity the organisation is planning to pay me monthly, it should be enough? It would be for a bit less than a year

In that same timeline I might officially know If i become a phd student or not at the same institution where I should be doing my internship depending on the MEXT results (university recommendation)

If I enter at first because of the internship, would I need to change visas after? Can I do both simultaneously?

Sorry, i just don't see much info related to internships online so unsure how it works in such scenario

Thank you!