I think I SOLVED this. READ BELOW
I bought the iPhone like 2 months ago, and tried everything, believe me everything. I even have like 10 apps that take photos and compared everything. I'll here write a "guide" to kind of solve this nightmare.
(However, APPLE SHOULD KNOW THIS IS NOT OK. Clearly people who don't like taking photos don't care about the camera, but we, who like taking selfies sometimes, are so disappointed).
Well.
1- At the settings, search "View full HDR". Let me explain: this is an option that comes by default activated. I thought this did nothing but it actually does. A month ago I tried that solution but I didn't like it. Today I understood... View Full HDR is just a thing made for the native iPhone Photos App. So, actually the horrible selfie you are seeing in the photos app is "not that horrible" if you find the photo via instagram's photos gallery. (But it's still horrible). First step is deactivating View full HDR.
Test it yourself like this: go to a place in your house with a nice light, (not direct sun) but a sunny day maybe. Take a selfie, and a milisecond after that, take a screenshot also.
If you have View full HDR activated, the screenshot and the photo will look VERY different. But if you deactivate it, it will start looking a little better.
Take into account that, since this is as I said, something for the Photos App; deactivating it will show photos the way they would actually upload in instagram. (So, for me, it's a good idea to deactivate it) (Activate it just for landscapes).
2- OK, now that you deactivated the fullHDR, also, let's adjust the exposure on the camera app. Go to the cam, and touch the little triangle. Then touch de + - icon and put more or less +0,7
(The idea is having this forever, so in the configuration of the iPhone camera you should activate Preserve configuration of Exposure Adjustment).
This will take more lively photos and your skin won't look grey.
3- Ok now that we have done those steps, go to settings again and in Camera/formats activate ProRaw control.
Now, before taking the selfie, make sure you hit the button "RAW". (You can also preserve this in settings).
We're done.
Conclusion: Now take selfies like that:
RAW + no view full hdr + exposure 0,7
This way, the photos will look very veryyy similar to what you see before taking them.
You may try comparing your "fixed selfies" to selfies took in Instagram. Both of them may now look nice.
The instagram ones a little darker maybe, because in ur photos app you have a little more exposure.
OPTIONAL WORKAROUNDS
a- Take videos instead of selfies
b- Use a third party app. I like Yamera
c- Take selfies in Portrait mode and put studio light. Modify the amount to your wish.
d- Take selfies without doing anything I said, but after taking them, put edit, and push the button above on the left, the one that looks like an hexagon so you can add "studio light" to your pic. This will make it look nicer.
NICE APPS TO HAVE and edit the photos a little more (Strongly suggested)
Lightroom, Peach, SODA
APPLE: PLEASE JUST MAKE THE SELFIES LOOK LIKE THEY LOOK ON THE SCREEN BEFORE TAKING THEM.