Sounds like you're a little confused- there's no such thing as "cereal" and "grains." There are cereal grains, which can be fed straight (either processed or unprocessed), mixed into a textured feed (that would be straight grain added to other ingredients and usually combined with molasses to keep it all together), or pelleted (in which case it's so processed that you can't actually see/identify the grains anymore). There are also pelleted feeds that don't contain grains.
For any horse, the main components of a diet will be forage, hard feeds, and supplements.
Forage is always the basis: high fiber feeds like hay or pasture. For horses in maintenance through moderate workloads, forage will be the vast majority of their diet.
Hard feeds are things like straight grains, textured feed, and/or pellets. They can be added for extra energy, general nutrition (to balance out nutritionally deficiencies of forage), or both. Cereal grains have a lot of energy, but very little nutrition.
Supplements are powders/pellets fed in small quantities to fill very specific nutritional needs, such as to support gut health (probiotics), joint health, etc.
Now back to the diet of racehorses versus your "standard" horse. A "standard" diet could be 15-20 lbs of hay daily. It may or may not include any additional hard feed. Racehorses are typically fed very high energy diets to support their very strenuous workload. They burn off more calories than they can ingest through forage alone and require some type of hard feed. There is a tradeoff there- many high energy feeds are also high in sugar & starch, which horses don't digest well. Feeding large amount of sugary, starchy feeds may provide energy, but it will also significantly increase the risk of ulcers (to which horses are very susceptible).