Joint letters are created by using Zero Width Joiner (ZWJ), a shared character in Unicode to form joint letters / ligatures.
Unlike some other languages, joint letters in Sinhala are optional, i.e., I can write "ksha" with or without a join. Therefore, Sinhala Unicode allows both forms.
Examples:
Please note that the end user doesn't need to know anything about ZWJ and other technicalities. Keyboard drivers generate proper sequences.