Interior design along with Architecture, Planning, Landscape , Urban Design, form core professions that help shape the cities and environments we live in. Within each of these professions you will find firms that specialize more than one of these design services, and firms that specialize in a certain sector (such as hospitals, or airports, hotels/hospitality, retail, restaurants, corporate offices.. etc). Most mid- large size building projects have an interior design component. So you will find that many large architecture firms have an interior design department.
As an interior designer you could work for a firm that design the interiors for a range of project types (most do more than a few), or something very specialized ( healthcare facilities, airports ).
You will design and develop drawings and presentations for new projects, select materials, finishes, create custom furnishings, coordinate with other consultants ( such as architects, landscape architects, lighting designers), ensure your designs meet local code and regulations and supervise construction on site.
Typically you start with an internship, work as a designer and graduate to managing project teams.
If you're interested in design and construction, this is an interesting profession to get into.