When you create things like a texture or a game, copyright laws prevent others from doing most things with it. As the "copyright holder", you can wave most of those restrictions to the benefit of others using a license. Free Software is software where people have the right to use it for anything, inspect it (both legally and practically via source code), modify it, and share it (both as-is or modified). To avoid the confusion about "free" in the English language meaning either $0 or freedom, Free Software can also be referred to as "libre". The opposite, where the rights of others are restricted is called "proprietary".
Unity is a proprietary game engine, which means that even if you give people rights over your game using a libre licence, the game will never be libre, since it requires proprietary code to run. However, as you say that you have only just started creating your game, you can still easily switch to another game engine. I would personally recommend you Godot, a libre game engine in active development, that can do both 2D and 3D. Your 3D models should be compatible, or easily converted using Blender (open->export). For the cards, you can use Sprite3Ds and Label3Ds.
As the design of Uno cards is proprietary (as far as I know), you should make your own or find some ready made. This is true for libre and proprietary games. Copyright law makes things like the preview of your video containing this artwork illegal. opengameart.org hosts many libre game "assets" for you to use. You should also look at the licenses of other things that you use, like the 3D models in your preview. If something does not specify any license, it is pretty much always proprietary.
There are also different licenses, many of which say the same thing with different words. You should pick one, and make sure that anything you include in your game has a license that is compatible with yours. The two main ones I recommend are GPL3-or-later and Apache2. The GPL helps fight against copyright, by requiring that people that share the software do so under the same libre license (as well as fixing many loopholes). This kind of license is called "copyleft", and also serves as a better substitute for proprietary "non-commercial" clauses. If you have issues with the idea of copyleft, you can use Apache2 instead, which is also well written and patent-aware.
You also used a Youtube link to showcase your project. Youtube uses a large amount of proprietary JavaScript running in people's browser to function. While not all people making libre software mind this, you can avoid it easily by hosting your videos on a PeerTube instance. PeerTube is a libre video hosting platform, that also happens to be federated for additional scalability and freedom. You can choose a PeerTube instance of your liking using instances.joinpeertube.org, host your videos there, and everyone on a federated instance will be able to see it. If you still insist on using Youtube, you can also replace the raw link with a proxied one, via Nvidious or Piped. Those software run by many volunteers allow people to watch Youtube videos without exposing themselves to any proprietary JavaScript.
To learn more about Free Software, and why it is so important, you can go the site of the Free Software Foundation.
