Without sharing too many details , Google goes by what the airlines "offer" Google to list. There are two buckets, one, which Google prioritizes, is complied based on the offers the airlines (or OTA) make to Google. The second (which you usually find under "other options" is composed of the search results that are independent of the offer -- they go by the usual criteria such as the distance from the airports selected, connections, duration, cost, the authenticity of the vendor, etc..