Adding Custom Fonts: Normal, Bold, & Italic

I want my apps to look as similar as possible on all platforms, and this obviously means including any fonts I use with app. I have read everything on the following page:
http://saplin.blogspot.com/2018/12/xamarinforms-custom-fonts-with-android.html
However, I am somewhat confused about what will happen or what to do about things such as Bold, Italic, and other font modifications. How does the app know which file to use for things like this? As much as I would hate needing to change the font I use for all my apps, I would actually prefer to see a list of what fonts are natively available for all platforms so that I could just change that rather than include the font with every app. However, back to my original question, how does the app decide which *.ttf file to use for the different font modifications, and what do I need to do about it? Thanks.