You can filter products export by Collection.
Put the collection Title, ID or collection Handle in the filter box – you will export only products that belong to this collection.
You can get the collection ID from the URL when you open the Collection page in the Admin. Or you can get the collection Handle from the SEO part in the Admin, or from the collection page URL on your store public site. Also, you can get collection ID or Handle by exporting Collections – then you can see those in the exported file.