The world of interiors is very large indeed, and goes a long way passed interior decoration, although that has its place. Interiors: commercial, domestic, hospitality, corporate, entertainment … anywhere that is 'inside', and even then it may not stop there.
Interior designers can create ambiences for many occasions and many situations.
An interesting aspect of retail and restaurant spaces is that they are sometimes refurbished every so often, just to keep them fresh and appealing.
As with architects, all you need is an address book full of valued clients who like your work.