Does “Aria the Scarlet Ammo” manga continue off from the anime, or does it turn into the spinoff?
Actually, there are more than 1 manga series: 


Hidan no Aria: started as a light novel, got adapted into manga &amp; anime.
Hidan no Aria AA: started as spin-off manga, got adapted into anime &amp; light novel.
Hidan no Aria-chan: a 4-koma spin-off manga.


So, to answer the question: yes, Hidan no Aria manga continues from the anime, which is an adaptation to the original light novel. The anime ends at volume 3 of the light novel, which is around volume 8-9 of the manga.



Note: Hidan no Aria manga continues as Hidan no Aria - Shiden no Majo after volume 16.