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: This is because her dad will be away for weeks at a time due to work, and we want to make sure I am able to take care of everything legally without any problems.

(FYI again just because sometimes people don't see it, her biological mom is not in the picture at all)
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