Yes, that's usually how it's done. You just have to make sure the licenses of the assets you are using are compatible. Some license, although royalty free, are not compatible with open source licenses (for example, a song might be royalty free to use on a project, but you aren't allowed to redistribute the song, only use it embedded in your project. So, you have to check for that).
Most of the time, you can check if licenses are compatible by googling that info.