Please help clarify bash this common found code

[Budi]

Please help clarify bash this common yet doubtful line

So often read or found a variable definition (or/and declaration) is
at the start preceding every command/code, one terminated by shel
terminator the other not

This merely illustration, what is the definitive function/behaviour on
e.g. each of

FOO=bar ; find /usr

and

FOO=bar  make Makefile

Please crystal clearly elaborate it