Thus might get more response in the iOS forum however will attempt an answer.
If using the on screen keyboard, press and hold the space bar which puts it into trackpad mode. Move finger around screen for precise cursor positioning. You’ll know it’s in trackpad mode when the letters disappear on the keyboard.
If the smart keyboard is attached, two fingers in the text area turns on trackpad mode.
If looking to select words, lines etc, hold finger on screen on one of the words you want to select, wait until the magnifier shows, the release and you should be presented with selec, select all etc. use select which will select that word, then grab one of the handles at each end and drag to select whatever additional text you want. From time to time it does decide to play silly sods and seems impossible to control selection (most often in Safari web pages). Closing the browser and reopening seems to help for me (as does using Chrome)