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I'm sure CGTrader Support will follow up and confirm this, but it should be perfectly fine to use Royalty-Free licensed items for the purpose you've indicated here. As long as you are not reselling the items in any way or claiming them as your own creation, you should be free to utilize them as backgrounds, props, etc. in any animation or video production, even if the videos are monetized. Provided, of course, you've paid for the rights (purchased the models). To my knowledge, this is part of the purpose of that licensing type.
I don't think anyone can speak for the free models you've downloaded, if they weren't downloaded from here, but purchased models with Royalty Free licensing are intended for that type of use.
