just vs only

Just refers to exactness. Only refers to uniqueness.
In many cases, they are interchangeable. But not always.

Just can indicate a short period of time when something happened.
I just washed the floor, and it’s still wet.
= I washed the floor a few minutes ago.

Only has a meaning of one.
I have just one brother.
= I have only one brother.

I thought you were hungry, but you ate just half of your sandwich.

just = exactly
This is just what I wanted to do.
He's just two dollars.

only = limited to
There are only 100 tigers alive.