a) atma weapons are the relic zenith upgrade ( stats are same as zenith)
b) ???
c) u need 12 atmas to upgrade... each atma is dropped from fates in specific areas, not specific fates! for example atma of the maiden is dropped in central shroud by any fate... oh and RNG is involved here so prepare to waste your time
d) u need atma drops in order to upgrade the weapon
e) u need any zenith weapon equipped to have a chance (lmao) to get an atma and ofc quest "Up in Arms" which u can unlock in an npc near Gerolt.

P.S: if you like grinding do this otherwise do it later or never!