Nurses require licensure in the state that they are employed in. To hire a nurse in... say Illinois, you need to have someone licensed as a nurse in Illinois first.

How hard is it to get a licence in Illionois, if you have a licence in some other state already?

Submit an application to take the next state licensing examination.

There are some states that have a compact and getting a multi-state license for those compact states will let you switch practice in multiple states - https://www.ncsbn.org/nurse-licensure-compact.htm

However, Illinois isn't part of that compact so its a new exam for anyone licensed elsewhere.

Aside from all of this... (as related to H1Bs) - nursing isn't a specialized occupation and doesn't qualify for the H1B. A specialist nurse, however, may. https://www.immi-usa.com/h1b-visa/h-1b-nurses/ ... and that is a different license (in addition to) the RN.