By default, a new offer includes no products. You choose what's covered in the Products tab.
Click Add Products, then choose from:
Products: select individual products one by one.
Collections: apply the offer to every product in a collection.
Tags: apply the offer to any product carrying a specific tag.
You can combine all three. Once added, the All sources view lists everything currently included in the offer. Use the sort icon in the top right to reorder the list.
Tip: using a tag (like notify-eligible) instead of hand picking products means new products get automatically included in the offer as soon as you tag them, without editing the offer again.