Selecting a Favorite Format is only temporary. It gives you a way to quickly change the way you're looking at the current folder to something you wouldn't normally want. The opposite of what you want here.
If you want to change the default format for all folders (not just Downloads), then follow this guide:
If you only want to change the format for the Downloads folder, the steps are similar, but you should choose the option to only save the format for the current path.
You can see the list of all formats under Preferences if you want to review things, and hovering over the format lock on the status bar will show a tooltip telling you where the current format comes from.
The video at the bottom of that guide is probably also worth watching, both for what you're having trouble with as well as to learn some advanced column resizing options that can make it so a single setup adapts well to both single and dual-pane windows.