If I buy data booster for USA, do l need to turn on data roaming when I get to USA to start using?

Yes, your data won’t work in other countries if data roaming is turned off.

Will there be any welcome message to indicate I can start roaming?

Yes you should get a welcome message as soon as you connect to a non Canadian tower.

Can I use data with any of the major providers like AT&T or T-mobile without extra costs?

Koodo will automatically select the best connection from the carriers they are partnered with. This will happen automatically and doesn’t include every US carrier. The only costs occurred to you are those of the boosters you buy.

Will I be charged any hidden roaming pay per use fees?

There are no pay per use fees on prepaid. You only get charged for things you pre pay for.

Will there be additional charges added if I go above limit?

Same a previous question. No additional charges. Your phone will just b unable to be used past the limits of the boosters you buy.

Prepaid roaming in the US can be hit-and-miss. You will at least need a relatively modern phone that supports VoLTE. Don’t go overboard on purchasing boosters in case you have a problem.