Power of Attorney vs Guardianship? ?

I will most likely have my boyfriends daughter by myself for a year (she is not mine biologically, her real mom is not in the picture and has lost legal rights...just fyi)

In order to put her in school, take her to the doctor etc. Do I need just a POA or guardianship? And what is the difference?
Update: It is because of work schedules and location. We will be away weeks at a time from her dad while he is at work, and we want to make sure I can take care of everything.

Just fyi again (because sometimes people don't see it). Her moms not in the picture at all legally.
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