Human Resources is the gateway to employment. There are a bazillion Federal regulations that impact hiring - particularly for health care providers -- and HR is responsible for compliance.
The first step is to determine whether there are any open positions for which you are qualified. If they don't have any openings, they cannot hire you. The number & type (full time, part time, prn) of positions is determined by the organization's labor budget. No matter how much they like you, they are not going to create a new position just for you.
Then, submit your application & resume for the position(s) you have identified. If you are dealing with an online application system, you may have to create a separate application for each position you are interested in. If it is a traditional paper-based system, be sure to include the position information in your cover letter -- "I am applying for the XYZ position listed in _________________(where you saw the listing) Good online systems will have a feature that lets you check and see the status of your application... they will also have some sort of automated acknowledgement. Fair warning... some online systems are really bad and difficult to use. If they use a traditional paper-based system, you will need to check back with the HR department to check on the status of your application.
When they receive an application, the HR department compares the applicant's qualifications to those listed in the position. IF there is a good match, they will then forward it & the resume to the hiring manager. If not, they won't. So, if the position says 'experienced' and you are a new grad - it is highly unlikely that the hiring manager will even see your application or resume.
If the hiring manager wants to interview you, they will schedule an interview with you. The hiring manager will decide who should be hired... and let HR know. Then HR has to run background checks and determine starting salary based upon their compensation structure.... WHEW!
So - the person who 'chooses' is the hiring manager for that department. BUT - HR controls all the processes & does the background checks....