Smart Text Selection tool.
When you’re highlighting what it detects to be a measurement, it will include both the number and the unit
Then you’ll see a Convert option, next to Cut, Copy, and Paste and then it will hand it over to the Google app adding the word “convert” to the search term. You will then have to select the unit you want to convert it to.
This seems to work on most units including length, weight, volume, as well as currency