In US English, transit is only used as a verb, not a noun. This is why it is only correct to say ‘I had a connecting flight in Hong Kong on the way to India.’ And not ‘I had a transit in Hong Kong on the way to India.’

‘Transit’ has many noun meanings. It also has a meaning related to ‘connection’. I took a connecting flight in Hong Kong. I had a connection in Hong Kong. (This isn’t necessarily air travel but most likely would be perceived as such) To be ‘in transit’ means to briefly stop somewhere and then continue on the same plane. I was in transit in Hong Kong for half an hour on my way to India, but otherwise have never been to the city. (My plane landed there, then the same plane took off) As far as saying ‘I had a transit’ to describe the above scenario, I wouldn’t. I’d say ‘I had a stopover in Hong Kong’ instead.