interrogative particle
歟 is a classical interrogative particle used at the end of a sentence to mark a question or rhetorical question, similar to '?' or 'is it not?'.
歟 is a phono-semantic compound: 欠 (open mouth, related to breath or utterance) combined with 與, which likely provides the sound. The character was used in classical Chinese as a sentence-final question particle.
The right side 欠 suggests an open mouth asking a question, while the left side 與 looks like a complex set of hands offering something. Imagine someone holding out an object with both hands and asking, 'Is this it?'
For ヨ, imagine a person asking a question and getting a simple 'Yo!' in response, as if calling out for attention: 'Yo!' -> ヨ.