I know that their original sounds are equivalent to Glagolitic Ⱏ and Ⱐ, but are those Glagolitic written symbols where the Cyrillic written symbols came from? Or did the symbols Ъ and Ь arise independently and get assigned to the sounds of Ⱏ and Ⱐ?
It's obvious, Cyrillic came from Glagolitic.
There are three Bolgarian (Christian church language) signs: ъ [er], ы [ery], ь [er'] and they stand together as in Glagolitic as Cyrillic. Those are not letters but just signs.
If you ask the origin of the symbol, it seems to be a medieval form of Greek "ε". Look this pic of a manuscript. It's in the first line of black letters, and resembles the number 6. It reads "Τω καιρώ εκείνω ...". The second occurence is a combination of ε and ι. http://www.xanthi.ilsp.gr/istos/images/xeirografa/foto/xeirografa_045.jpg
It's obvious, Cyrillic came from Glagolitic.
There are three Bolgarian (Christian church language) signs: ъ [er], ы [ery], ь [er'] and they stand together as in Glagolitic as Cyrillic. Those are not letters but just signs.