The old Mercury, Gemini, and Apollo capsules were one flight vehicles. To be best of my knowledge, virtually nothing from one mission was reflown on a subsequent one. That's one way of doing things.

Another way is to design the vehicle so that parts of it are reusable and parts are expendable. For example, if design studies show that the main cost is in the internal systems, then it might be worthwhile to design the capsule shell to be thrown away but to be able to remove the internal systems for use on subsequent missions.

The final alternative is to design the capsule to be fully reusable. The heat shielding would either be replaceable or perhaps use some more advance form of thermal protection tiles. The internal systems could be designed for easy maintenance access (what a concept!) to allow for removal and servicing as necessary.

It's possible to design a fully reusable capsule. The real question is whether doing so is economically viable. If it turns out that the development and refurbishing cost is too high, then a partially reusable or even completely expendable capsule may make more sense. There's also safety factors to consider. A fully expendable capsule requires an very extensive checkout of every component before flight because all systems are new. A partially reusable capsule might not require as extensive a checkout because some of the components would be proven hardware (and software). Conceptually, a fully reusable vehicle might not require much more servicing between missions than is required by an airliner. In practice, it hasn't worked out that way. If building in reusability ends up in creating too much complexity, then safety might actually be lowered by going that route.