IO lines are used in critical care nursing as well - ICU for example. Also, most (not all but most) IO lines are placed in the field prior to arrival in the emergency department and are usually done by the EMT-B/EMT-P that is on scene.
Typically ED nurses see the patient, treat the emergency condition and either send them home or admit them to a unit. They don't look at the signs and symptoms long term they just are the immediate patient care needs and stabilize them to determine what needs to be done NOW.
Med Surg nurses are more reviewing the history of the patient in depth and observing the patient and how they are presenting based on their admitting diagnosis. This is why clinicals are done in these settings - so students can review and see the big picture and how it seems like the book.
As many will tell you, you really do just "know" and there's no explaining how you know. Try researching RN opportunities in your area. There are nursing homes, skilled nursing facilities, stand alone surgical centers, schools
, long term acute care centers...basically there are tons of avenues of nursing. Don't limit yourself to just hospital nursing if you aren't sure yet.
Also, keep in mind - not everyone has the feeling of knowing what area of nursing they will find to be their niche. Have you been through your pediatrics or obstetrics rotations? Have you shadowed the other hospital units - ICU, oncology, etc?
Just focus on getting through school and soaking up all the knowledge and learning experiences! Enjoy the time you get to float as a CNA.