Are you looking for this?:
https://www.decathlon.co.uk/watertight-phone-pouch-id_8480983.html
For me it has never been an option. I don't like weight moving all day on my neck, or in a trouser pocket.
I'd suggest a seperate bumbag. Cheap onces are sold in Decathlon. You can also have the powerbank, some money and a snack inside and if you get a bigger size a small water bottle will also fit. The benefit: it stays with you even if you put your pack off.
Another option are bags attached to your belt or to some straps of your backpack. I had neopren pocket for a Nokia years ago (much less weight and size than current phones). It worked, but I left the phone sometimes with the backpack.
If you can reach any pocket on your backpack and open/close it without getting the pack off, it's something you could try. Newer backpacks tend to have more pockets than the ones sold 10 years ago. Time for a change?