Both composed substances and mixtures have more than type of atoms.
However, there are some difference between them both.
1)composed substances=mix chemically, mixture=mix physically
Composed substances are usually mix together through chemical process just like burning, titration and redox reaction.
While mixture are mix together through physical process like dissolving and even putting together.
2)Composed substances and mixture properties
Composed substances have different properties with it original atoms.
i.e iron sulphide do not attarct to magnet although iron is magnetic.
While mixture have the same properties with its original atoms.
i.e. a pile of iron filling and sulphur powder. The iron will still attarct to magnet and the sulphur don't
3)Bond between atoms
In composed substances, the atoms are bonded with either ionic bond or covelent bond. i.e the hydrogen and oxygen atoms in a water molecule have colevent bonds between them.
In mixture, the atoms do not bond together. i.e oxygen gas, nitrogen gas, carbon dioxide gas in the air do not have bond between them as they flows freely in the air.
They are also a lot more differences between these both such as melting point, existing states of matter, conductivity of electricity and heat etc. I hope this helps.