Disclaimer: the following is my opinion.
It shouldn’t be that confusing.
When there is no licence that explains what you are allowed to do … do not use it. It means you have no explicit permission.
When there is a licence, check what permissions it provides to you and what you are allowed to do.
You are not allowed to give it to someone else. You can tell other where they can get it too, but you are not allowed to distribute it.
So no you can not ship it with your game regardless if you sell it or not.
You are not allowed to sell it. It does not necessarily mean you are allowed to give it away for free (it might be but it is better to have an explicit permission).
Btw. commercial - does not mean selling. It means you act commercial and the asset is used in this process.
I do not think this is a right/permission. It also depends on the local laws. You need to check the context.
Free to download
… you are allowed to download it (from there). It might even mean you can download it without paying anything. Be aware this is “download only” it does not mean anything else.
Free to use
This is a very general permission. Typically “use” means: read, modifiy, distribute, sell, and other uses.
Be aware such statements are often just a small part of the licence (like an ad which is not explaining all the little details - especially the negative onces).
Often the permissions are restricted. E.g. as you wrote - “Credit should be given”. This is not automatically. You do not need to show credits if it is not required by the licences (but you can).
Often assets are distributed with a licence and you need to redistribute the with this licence too.
Be aware your conditions might change over time which might change the permissions you have. Examples:
- exceeding a certain revenue (free until you earn 10000$)
- exceeding the number of permitted distributions (permitted to distribute it 10 times)
- exceeding a time limit (14-days test period)
- not beeing a student anymore
- not beeing an employee, member etc.
- not paying a fee
Some licences are incompatible with others. That means you are not allowed to distribute assets in certain ways (e.g. bundling assets with the blenderplayer as this has to be distributed under GPL).
Also keep in mind, the sources you get your assets from might change their licence in future. While this has no effect on the assets you already have (unless it is stated in the current licence) you should document the current licence rather than relying on a third party website - which will change.
For your safety I strongly recommend you document:
- Where did you get the asset from
- What asset it is and where you have it (path + file name)
- The licence it was offered at that time
- The conditions of that licence
This way you are able to check if you still meat the conditions of the licence when you distribute your work. You should also know what your distribution contains and what licences apply. And you should know what licence you use.
Btw. this documentation is for you. It is part of your project documentation. You do not need to distribute it.
As private person I guess you do not think much about licences. In most cases nobody notices when you do something that is not permitted. When it comes to distribution you make visible what you did with the the assets. So you should at least think about it.
Unfortunatly I think this confuses you much more right now .