How to type in Sinhala or Tamil on your phone and computer
If you type directly in Unicode, you never need to convert anything for the web, WhatsApp or email.
On Android
Open your keyboard's settings, add a language and choose Sinhala or Tamil. Or install a keyboard app that offers Sinhala or Tamil. Many people prefer one with phonetic typing, where you type in English letters and it suggests the matching letters.
On iPhone
Go to Settings, then General, then Keyboard, then Keyboards, and add a new keyboard. Tamil is available as a built-in option. For Sinhala, check what your version of iOS offers.
On Windows
Open Settings, go to Time and language, then Language and region, and add Sinhala or Tamil. You can then switch keyboards from the taskbar, or with the Windows key and the space bar.
When you need an old font anyway
Type in Unicode first, then use the converters to get Bamini, STMZH or FM text when a printer asks for it. That is far less painful than typing in a legacy font from the start.