Correction - #2 They rename it to Portrait mode 😌
Regarding #1, you just change the aspect ratio to change the resolution. It's that simple and the resolution is exactly the same as before. And this is more simplified and ready to use. I don't have to go into settings each time to change it. I just tap aspect ratio now and done. Much more user friendly in this case and quicker to get the resolution you want and use.
You can test this by taking a pic for each aspect ratio. Then look at the pixels for each. If you compare it to the old resolution settings of the Note 10+ it's exactly the same.
#2 see above
#3 not sure what your are trying to say. Picture in Picture is still working with the Note 10+.
Please note that the features and specifications of the device may vary from one version to another version.
Go to Camera app> Tap on Settings symbo> Open the photo from the Camera Gallery> Tap on Edit symbol> Tap on three dots in the top right corner> Select Resize.
Here, you can also select the resolution of photos like Free, 1:1, 4:3, 16:9, Full.
As mentioned by Tallman, it is renamed to Portrait mode on Samsung devices. Please Tap on the MORE option to get the different camera modes.