Community created content is not free to use - that doesn’t necessarily mean you have to pay for it however.
Most of the time just getting permission is enough. Just a few days ago I gave someone permission to use a UGC item I made - they simply asked via a direct message, and I gave them permission.
This is super easy and should be done for all asset types to respect creators and protect yourself, because failing to do so can open up issues such as DMCA takedowns and strikes.