The problem is that an exported Unity project does not include the code necessary for importing models in 3rd party file formats like OBJ into the internal asset format of the game engine. That means importing raw .obj files at runtime would require your own implementation for parsing the binary file format (here is the specification) and turning it into a Mesh object. This couldIt's not actually that complicated compared to other 3d file formats out there, but would still be quite a lot of work, depending on how much of the format you need to support.
However, it would be much easier if you packed those asset3d models into Asset Bundles using the Unity engine. An asset bundle is a packed archive of already imported and game-ready Unity assets. These assets can be just the models, or even complete prefabs with multiple components including MeshFilters which reference those models.
You can then download those asset bundles at runtime with the method UnityWebRequestAssetBundle.GetAssetBundle(url). You can then extract the assets from that modelasset bundle via the methods of the AssetBundle class or via Addressables.