KineMaster and the assets on the Asset Store can be used for any purpose, no matter if it's personal or commercial (for monetization), without any concern about copyright infringement.
Some assets are copyrighted by KineMaster, and are licensed to KineMaster users for unlimited use as far as they are used with KineMaster.
The other assets are copyrighted by 3rd-parties, but, under contract with KineMaster or under its own end-user license, thus KineMaster users are allowed to use them with KineMaster unlimitedly.
All that you have to know is to just use the assets only with KineMaster. That is, if you use KineMaster assets seperately to KineMaster, it conflicts with KineMaster Terms of Service, and violates the copyright laws.