My mother's upgrading from a Huawei P9 to an iPhone she received as a gift (don't know which model it is).
She doesn't know how to use iOS and I'm finding it difficult to teach her, since I don't know how it works either, so I was wondering if it was possible to install some version of Android on it.
The other replies answered your question already, but this may solve your little "problem" here: Apple offers free in-person training sessions on how to use their products. They're intended precisely for non-technical people like your mother.
So, if you don't want to be the person teaching her how to use iOS, you could look into getting her to attend these sessions instead.
You can fault Apple for many things but this offer of theirs is just great on every level IMHO.
Updoot for this - my ex-mother-in-law got a lot out of this by both taking classes and dropping in to ask questions. They were always helpful and polite, and spent whatever time she needed