Depends on the bike and the course.

What distance race?

For training, I have a BTA bottle and one on the downtube. On race day I remove the bottle and cage from the down tube for the aero savings, but keep the BTA bottle. If it is a 70.3 or longer, I add a second bottle behind the seat.

Other than that, I throw on my race wheels and give the chain/drivetrain a good cleaning and lube.

I would think that it would be a really silly idea to change the saddle or anything else for race day.

ETA: as far as your question about where people carry food: I carry gels in my kit and run a dilute infinit in the BTA bottle. For longer races, I keep a concentrated version of infinit in the bottle behind the seat. When I pass an aid station, I squirt a little bit of the concentrate into the BTA and then fill the rest of the way up with water.

You will get a lot of answers on this. First and foremost will be "what distance are you racing"? The answers will be far different it you are talking a sprint vs a full distance. Answers will also vary a lot if a person plans on using on-course nutrition instead of a personal blend. The more you can rely on the on-course stuff the more minimal you can go with what you carry. Below are my general options:

For a sprint I'll just have a single bottle in a zip-tied cage BTA with water or maybe a light mix of Tailwind/Gatorade/etc. My flat kit will be in a bento bag. No gels or food.

For a 1/2 IM I'll up it to two bottles (one BTA and another probably behind the saddle), flat kit moved to under the saddle, and some gels and/or bonk breaker in the bento bag that previously had the flat kit stuff. I'll grab extra bottles as needed from an aid station (as I will use whatever drink they have on the course).

For a full IM I actually use pretty much the same as the 1/2 option. But I'll add an extra tube and CO2 to my special needs bag and take hand ups as I roll through aid stations. I might stuff some more food items in my bento bag or stash a few extra items in a pocket as running out of T1. Again, grabbing extra food items as I roll through an aid station.

The question you will want to figure out is how much you need to take in for the length of the bike portion of the race and add those calories and ounces up in the numerous ways to can make it happen. The bike isn't meant to be a smorgasbord. Live off the course if you can. If you have a gut that doesn't like the course options, you will no doubt need to carry more.

Let's say a 70.3

Bta bottle with 200 cal infinit, concentrate nutrition bottle with +/- 600 calories behind the seat, 2 gels in my kit. 4 salt tabs, CO2 cartridge, inflator, spare tube and patches in the bento, race wheels, clean drivetrain, tune a week out from the race

Keep it minimalist. Flat kit, BTA bottle, Concentrate bottle/gels or live off course gatorade, Refill at aid station.