There are basically 3 types of automotive primers in use. Primers cover bare metal, provide a surface to sand smooth, and give a uniform surface for paint to stick to.
A direct to metal primer is used to cover bare metal. This can be in the form of an etch primer, an epoxy primer, or a urethane primer. All three are designed to protect metal from rust, but do it in different ways.
An etch primer contains acid and sacrificial metals to accomplish adhesion and rust protection. The acid in the primer helps it stick to metal, and the sacrificial metal such as zinc, prevents the metal from rusting. Etch primer is not designed to be sanded, and does not build up the surface like a primer surfacer. It is usually light green or light gray in color.
An epoxy functions more like a glue. It is a very sticky primer that adheres to metal and prevents moisture from getting in. It is similar to an etch that it is not designed to be sanded, but needs to be lightly sanded before topcoating if it has been allowed to dry too long. Check the manufacturers product data sheet for specific dry times, because every product is different.
A primer surfacer is made to build up the surface of your repair. Surfacers are usually not applied over bare metal unless they specify that they can be applied direct to metal on the product data sheet. The area you are applying surfacer to must be sanded first. 2 to 3 coats will give you enough material to block sand. After applying, you sand it smooth to get it ready for paint. Modern primer surfacers are almost always urethane and 2k, meaning they require a hardener to dry.
Polyester surfacers, which are basically thinned out body filler, are popular for restorations because they build the surface several times thicker than urethane surfacers. The downside is that they are brittle and tend to chip easily. They have their place, but I wouldnt recommend it for a daily driver.
Primer sealers are used as a final step before applying color. They are designed not to be sanded. Most sealers are applied wet on wet, meaning you apply sealer, then apply color directly on top after the solvents flash off. They typically have a drying window where you can safely recoat. if you exceed the recoat window, you will have to sand the sealer before applying color. If you are within the window, you can put color directly over the sealer without sanding. The benefit of sealer is that it will fill light scratches and provide a uniform color for paint to go over. Using a sealer that is a similar color to your color coat will allow you to use less material on your color coat.
This guide will help you understand the basics, but please refer to the product data sheet before using an unfamiliar product.