So I think you are asking about the F1, F2 & F3 keys. The default behaviour is to select your connected device.
When using with a Windows pc, You need to download the Logitech options software. This will allow you to set the f-lock so that the F keys function as regular F keys. If not, you will need to press and hold the fn key (bottom left) to use the F row as Fkeys. Poor design by Logitech in my opinion.
I have this keyboard for about 3 weeks now I like it other than this silly setup to use the F1 keys
Hope that helps.