Completely OT, but which scientific keyboard did you choose? I was somewhat expecting to find a scientific layout for Gboard.

I just use Google for obscure Unicode characters or emojis. Usually there's a result I can copy / paste.

https://www.google.com/search?q=2+subscript+unicode

SciKey on the iPhone.

The free version works OK, and for 99p it can do something fancy, I’m not sure what though.

Subscripts especially for CO₂, has been irksome for me for ages.