I write stuff on the phone. Soo... ?
It should work the same way on a mobile device. (Just tap instead of click.)
That's obviously not quite exact but there should be handles that allow you to adjust the highlighted area.
(This is how it works in general. As I don't use CHYOA on mobile, I can't say for sure.)
Another possibility is to deactivate RTE and write plain Markdown.
To do that, you have to deactivate the option
"Use the rich text editor to create and edit chapters"
on your
Settings page. (Note: Do not switch that option while you have a chapter open.)
With plain Markdown, writing
or
results in
bold
and
or
results in
italics
(The result is visible after you saved the chapter.)
I'm not sure if that is convenient on a mobile device as asterisks * and underscores _ might not be easily reachable depending on the virtual keyboard layout.
I think a user once mentioned that you could use a shortcut in the browser to switch to bold/italics and then switch back once you're done. Maybe there is something similar on mobile devices.