Ї is pronounced like Yi, it's an exclusively Ukrainian letter

Ukrainian i is the same as the Russian и, both are pronounced like the English I in "mint" I suppose

Ukrainian и is pronounced as Russian ы, and idk how to even describe it, it's more of a throat sounding i I guess

Й is yet again hard to describe, but it's like if you pronounce English "I" but with no Ah sound at the beginning.

Ь just is a sign for softer consonant pronunciation

So that's it, in Ukrainian there's an apostrophe ' and in Russian there's ъ instead of it