Typically you create a single model with submeshes for the clothing and equipment. To customize, you enable and disable submeshes, and/or switch out materials.
For examples, look at Arteria3D’s One Base Male and One Base Female:
Or any of the Fantasy Horde series by Polygonmaker:
Weapons are frequently separate models. For example, say you have a separate pistol model. You can create an instance of the pistol and attach it as a child of the model’s hand bone. You might have an empty bone that’s a child of the hand that is reserved especially for attaching equipment. These are called attach points, and can be on the heads, belt, back, shins, head, or wherever you might regularly add equipment.