These words stand for 'or'

I am very confused because I don't know when to use each of them. Can someone please help me with this?

Thank You.

There is no difference between uses of "veya" and "ya da". You can safely use any of them whenever and wherever you need to use "or".

Thank You, Cyrano!

Are they subject to the vowel and consonant harmony rules?
Or remain constant whatever the word following them is?

Warmest regards

Thank You, Cyrano!

You are always so kind to answer questions!!!

My kindest regards,
Mella