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I think B1/B2 visa last of 10 years.

I guess the 30 Days visa are J visa ?

I think 10 years visas are for some "friendly" countries, others get 3 year visas if no 221g procedure was involved.

It's based on a subjective risk assessment. For example, middle aged Indians are usually issued 10 year visas to visit their children in the US.

I (Brazilian) had to get a J1 for a 3-month internship despite having a B1, my sister had a 6-month J1 for another exchange, so I guess it can vary

If it is work or education of any kind (internship falls into that bucket I guess), then B1/B2 won't work.