If you specify what you want, but also include a line such as “or equivalent approved”, then, the contractor can use equivalent alternatives, provided they meet with your approval.
Specifying it like this allows you to illustrate what you want it to look like, but also the quality of materials you expect. The builder will then be able to go to their preferred supplier or manufacturer to see if they do a similar product to propose to you. Make it clear that you are willing to consider their proposals, otherwise they won’t bother going to the effort to find alternatives.