Most of the Koreans 374 including the 14 others nationals have visa B1/ESTA.

Vast majority of the workers detained were mechanics installing production lines at the site, and were employed by a contractor.

If not installers, why Trump wants them to stay?

All these visa things are Gibberish Words Soup.

This is a huge factory 2,900 acre with an investment of USD$4.73 billion and going to employ 8,000 workers.
It hasn't even start operating!

Now Trump is trying to work a new "Visa" for these technicians to come back to USA, help set up & train workers to run the factory.
Which is why he was reluctant for them to fly back initially.

Korean companies would be "very hesitant" about investing in the US following the raid where workers were chains & handcuffs like slaves, South Korea's president said on Thursday.

"The situation is extremely bewildering," Lee added, while noting it is common practice for Korean firms to send workers to help set up overseas factories.

"If that's no longer allowed, establishing manufacturing facilities in the US will only become more difficult... making companies question whether it's worth doing at all," he added.