Sinhala font

You aren't able to run iOS apps on the Mac, and that app appears to be iOS/iPadOS only.

To type Sinhala on your Mac though using it's built-in international language abilities this might help:

https://discussions.apple.com/thread/8309295

There seems to be a download for MacOS here

https://www.helakuru.lk

Is that what you are looking for? Are you having trouble with it?

Apple only supports Unicode Sinhala. If you are interested in Legacy stuff, you need to use some other kind of equipment.