It depends on the type. Ridge-filling base coats improve the look of the surface instantly, because they fill in ridges as soon as they are applied. Hardeners and strengthening serums work on the nail's resilience, and nails grow slowly, so it is best to use them daily or as directed for several weeks before judging the result.
A hardener is applied in coats, like a polish, and reinforces the nail to reduce breakage; it is usually used a couple of times a week. A keratin serum has a light texture that absorbs instantly and focuses on restoring moisture and elasticity to the nail, so it can be applied several times a day.
It depends on the product. Ridge-filling base coats are designed to be worn alone or under polish, whereas some hardeners are formulated as a standalone treatment rather than a base. Before combining a treatment with colour, always check its directions.
Yes, that is what ridge-filling base coats are for: they fill in ridges and even out the nail surface in a single coat. You can wear them alone, with a natural semi-matte finish, or as a base so that your polish goes on more evenly and smoothly.
With hardeners that are applied in coats, the usual practice is to remove them completely before reapplying, so the product does not build up and the nail is clean for each application. Follow each manufacturer's directions, which are usually once or twice a week.