How to open question mark on android?

It depends first of all on the keyboard you use, which keyboard app.

I use GBoard, the official Google keyboard for Android.

Once you open the keyboard, if you see the close question mark key (?), just tap that key and leave your finger there for a moment, a second or two... you'll get options for new signs, one of them is the open question mark (?).

I'm not answering because I don't use android but you are honoured by the question. :-)

holding your finger on the question mark

Hold your finger on the question mark

Holding your finger on the question mark