This is an age old question, which comes first, design or product selection. That probably ultimately depends on your approach, and the type of home remodeling project you are undertaking. Below are examples of how one can come before the other, and a third example where they are happening simultaneously.
With larger and more complex projects, the design usually comes before the product selection phase. A good example is a kitchen remodel, which is one of the more complicated projects. You really need to design the layout of the new kitchen and the overall style and look of the kitchen. This will take time as you think about appliance locations, the amount of counter top space, the amount of storage and cabinet space, and whether the kitchen is purely functional or if there is a big entertaining component to it. This should all be far enough along before you really get into the product research phase.
A smaller project like putting in new hardwood floors in a room is more driven by a singular key product selection. You may have already decided you want hardwood floors in your master bedroom, and the design is really driven by which product you choose. Now there are probably elements like color and size of the planks to match with your existing furniture and paint, but nonetheless, the product selection is the key driver.
With some projects, design and product research are going on simultaneously. An example might be converting a room into a theater room. You may have design flexibility on whether the TV is wall mounted or on a piece of furniture. If you find the right TV cabinet you like [product selection], this might drive the design of the rest of the room with perhaps smaller sofas and chairs. If you go with a wall mount TV design, you might have more options in sizes of your furniture because you have more floor space.
It is important to have an awareness of your projects design and product selections are related to one another. Depending on your project, one could drive the other, or they could happen at the same time.