So your confusion arises from the idea that we need to explicitly point at the lst when changing it.
If we break it down we see that .append() is a function that acts on the list is it attached to. So lst.append(9) is saying "add a 9 to the end of lst". So by adding lst= to the front you are saying
lst="add a 9 to the end of lst" or lst=the action of appending the list.
This can be compared to a real life example.
We have a shoe as our object (the list) and tie as our function.
shoe.tie() would mean we are going to tie that shoe (in python syntax). That has all the info we need to know that we edited the shoe by tying the laces.
shoe=shoe.tie() is saying that the shoe is equal to the tying of said shoe. Shoe is something (a noun), while shoe.tie is an action (verb/function). So the whole thing errors out because you were trying to equate something with an action done to itself.