Globe key on M1 Macbook Air - what about the "fn" key ?

On the M1 Macbook Air, the bottom left key shows a Globe and an "fn".

When pressing the key, input source changes. However, what I want to do is (like with older models) press the "fn" key to toggle between e.g. brightness increase and F2 with the corresponding function key ...

Seems like there is no way to toggle those anymore, except for making a change in the System Preferences (which is not very practical if you want to toggle on a recurring basis).

What is the purpose of the "fn" indication on the Globe key then ?