Help in understanding first law of thermodynamics?

What my sir has explained me :
Change in energy = W=F(external force on body value) * displacement of wallls of container.

If walls of a container are not moved , then no mechanical work.

If you push wall , it doesn’t move which means no Change in energy.

Let us say if there a metal bottle.If I hit with a rod,there is displacement of walls of bottle I.e dents may be formed.

Change in internal energy = Macroscopic work(molecules inside the hammer which are hitting on the walls of container and pushing other molecules). * Microscopic work or heat transfer(It can be said as Temperature difference ).

Final formula I.e Change in internal energy = W+q.

When solving questions on this , if there is a piston and it is moved , the displacement it does it taken in terms of W.
What I did not understand form this :
Internal energy is K.E + P.E and it depends on amount of mass substance.J/kg.

My question ;
1)Where does amount of mass of substance is taken into consideration in 1st law.

2. I did not actually understand what he meant by macroscopic work?.
   Because in our textbooks , it written that macroscopic system is sth that is observable by our eyes.
   Is pushing other molecules wrong to say and instead write displacement by the walls ?