You don't find things like that in a keyboard app, you find them in the app you are typing text into (just like your laptop's keyboard doesn't have bold/underline/italic keys). So when typing here I can use the little line of controls above the text input window to change things like that.
So it does depend on what email client you use, but most will have such options. You may need to check the app's settings to turn on a "style toolbar" or similar, but 2 of the 3 email appsI use on my phone have such options (if ProtonMail has them they are well hidden, but that's not really what that app's about). If you see a toolbar that doesn't have the precise options you want maybe try scrolling it sidewards to see whether there are more options (one of my apps is like that).
If the email app you use doesn't do this and it's important to you, other apps are available (there's nothing special about the ones that manufacturer's bundle).