What do you want from your product photography? If you are seeking extremely high detail then you may want to go with a medium format camera. If it's high-end jewelry then you should also be considering your lens selection—common usage would be a 100mm macro and a 85mm beauty lens.
Product photography is a large genre of commercial photography. High-end product and well known brands still shoot for print but the more common output is digital. If shooting film is still an option for you then that opens up the possibilities of how you would answer "what is the best camera for product photography?" If large format film is an option, then shooting with a monorail 4x5 could elevate certain types of products, namely luxury products.
Car photography comes in many styles. If you're shooting commercially you should stick with a full-frame image sensor. If you're shooting for the love of it, having a full-frame may be more than you require, but you will love having it. Lenses are important with all forms of photography, including taking pictures of cars. The Canon 5D is a tried and true pro photographer's camera.
Real estate photography is a great way to earn a living with a camera. It is usually a requirement when shooting commercially to have a full-frame image sensor. 16mm-35mm lenses are common for interiors. Canon sells them on Amazon. You'll also need a sturdy tripod.
The larger the image sensor, the more information you can potentially capture. Using a camera with a full-frame image sensor or larger will benefit anyone working on portrait photography. Pairing a full-frame image sensor with an 85mm lens is a classic combination for taking photographic portraits. The 85mm lens is often referred to as the beauty lens. It has a some compression and does not distort shapes as you change your focal length.
The Canon 5D with a 85mm L glass lens is a great option for any photographer working in portraiture, whether amateur or professional. These two items are often sold separately. Check prices on the 5D here and the 85mm L here.