General education. Parts and media vary by model.
It starts at the main line
Water enters a home through one pipe, then branches to the bathrooms, kitchen, laundry and water heater. A whole-house system goes on that pipe before it branches. That's the entire idea, and it's why these systems aren't installed at a sink.
Fixtures that branch off before the system, often hose bibs and irrigation, don't get the filtered water. Many people prefer it that way, since there's no reason to run a garden hose through a filter.
Two common styles
Tank systems hold a bed of media inside a pressure tank. Water runs down through the bed and comes back up a center tube. Some of these backwash, meaning a valve periodically reverses the flow and rinses the bed to a drain.
Cartridge systems hold one or more replaceable filters in housings on a wall or a frame. They tend to take up less space and need no drain, and you change the cartridges on a schedule.
Why size matters
Two things have to work at once: enough flow for your house when several fixtures run together, and enough contact time between water and media for that media to do what it's designed for. A system that's too small can feel like a pressure problem, and it can also shorten the time water spends in the media.
That's why a good quote asks about bathrooms, home size and pipe size first. A price given without those answers is a guess.
What about claims?
Be skeptical of any broad statement about what a system takes out of your water, including from us. Performance depends on the specific model and its testing. Independent certifiers publish listings by model number. If someone makes a specific claim, ask for the listing and check that it covers the whole system, not only a part inside it.
