From the Settings -> Basic tab there is a section under "Startup Scene" called "Export type", here you can change the option from "Everything" to "Selected Items:"
This will allow you to choose one or more types to include in your playable from;
Scenes, Resources, Prefabs, Scriptable objects, Streaming Assets.
You can read more about this here.

From the Assets tab there a subsections for each of;
Assets, Textures, Sounds, Fonts, Meshes, Animations.
In each of these subsections you can choose to force-exclude (or force-include) assets of your choice (of that type).
You can read more about this here.

From the Code -> Exclusions tab you can select any scripts that you wish not to be included in your playable build, this is especially helpful for both thinning down your playable and avoiding build errors.
You can read more about this here.