it is said that the affect of harmful chemicals are most in the top carnivores but they are also eaten up by the decomposers. so my ? is that “are decomposers also get affected by those chemicals or not ? if yes then what is the quantity of that affect or if not then why not?

Biomagnification is aman-induced process.
Biomagnification occurs withnon-degradable chemicals/substances/pollutants only.
For example Pesticides, anti-knocking agents used in internal combustion engines, paints, artificial perfumes etc.
Biomagnification doesn’t occur with degradable chemicals/substances for example sugar, tea, cheese, milk etc. because they can be digested by (some) living organisms.