before cars we're people into horses and carriages? modifications and personalisation and tuning of the horse.?

Yes

The carriages could be highly customised, the tack and saddle for the horse could also show much finer workmanship with every bit of brass highly polished. The horse itself could be cleaner all round, better groomed and with neatly trimmed and oiled hooves.

Pimps could fit their horses with gold teeth and a diamond-studded aršehole. I may have just made up those last two...

Yes, before cars were invented people traveled in carriages pulled by horses or by train, but only those who could afford to. Those who couldn't afford to travel this way either walked or rode a bicycle.

No.

Before cars, the vast majority of people couldn't afford to own and maintain their own horses and carriages.

Maybe. One of my friends once pondered, "When kids had never heard of cars, did they push around sticks and say 'clip-clop' instead of 'vroom'?"

Not so much modification of the horses.

Rich folks would invest in beautiful carriage horses, though, and kept them groomed to perfection. A pair, or even a four-horse hitch, would be selected to match.

The carriages would be waxed and polished to perfection. Works of art that you can still see at shows.

Yes, those who could afford it would show them off.

You can see a little of this at an event where the Budweiser Clydesdales are showing. The horses, and the beer wagon.

Horses already came equipped with afterburner. Not much to tuning a horse other than healthcare. I imagine that buggies and carriages could probably be customized based upon the technology of the times. But there speed and acceleration off the starting line was limited to the horse's ability.

Yes, some people spent a lot of money personalising their carriage and tack and adding decorations to them.

When I was a Brooklyn, pre=WWII, Milk was delved by a horse & wagon ditto fruits * vegetables ditto knife sharping and ice for the ice box.

Yeah. There's an episode of The Waltons about that. Episode: The Pony Cart.
And if you watch Sherlock Holmes, it mentions the different carriages. Certain families had certain crests on their carriages, and wealthy women had carriages with step covers that attached to the door and that kept the step nice and clean when they opened the carriage door to get in and out so their shoes didn't get dirty.
I'm not sure about the horses