The first time I tried the query block I was lucky because I immediately found the button for switching query from URL to custom settings, one thing it's weird for me is the setting splitted in the block inspector and in the toolbar menu.

I also created a video where I build the home page with FSE, it’s in italian language, I’m sorry, but if you want you can see it in mute mode and 2x speed here the link https://youtu.be/q7Q0FZnSCSQ