One point of bearing, not a continuous line
A footer runs the whole length of a wall, spreading the building's weight out along a strip of undisturbed soil the same way a snowshoe spreads a person's weight over snow. A pier does the opposite. It carries one point load, almost always a post, straight down into the ground below it, the same way a table's weight rides down through four legs instead of through the tabletop. That difference is why the two words get confused. Both are concrete in the ground holding a building up, and from the surface a hole for a pier and a trench for a footer can look like the same kind of work before anyone has poured anything.
How a pier gets formed
Most piers get formed with a round cardboard tube set down into a hand dug or augured hole, filled and left to cure, then the tube gets stripped or peeled away once the concrete has set. Some go in without a tube at all, poured straight into a clean hole in firm ground where the soil itself holds the shape. Either way, the goal is the same, a clean column of concrete bearing on undisturbed soil rather than loose fill or backfilled dirt that has not had time to settle.
Depth is not optional
A shallow pier sitting above the frost line is exposed to the same ground movement that lifts a shallow footer, water in the soil expanding as it freezes and pushing up whatever sits on it. A pier that heaves even a little tips the post standing on it, and one corner of a building out of level tells on itself fast. How deep a pier needs to go for your site is a code question tied to the address, and we confirm it before anybody digs rather than guessing off a chart.
Piers and the floor are separate questions
A building standing on piers does not need a floor to hold it up. The posts carry the roof and the walls straight down into their own piers, so the ground in between can stay gravel or dirt for a season, or permanently, without the building caring. Once you plan to park something heavy or work on that floor, it becomes its own decision with its own thickness and base, unrelated to how the piers were built.