Tick Free Assets and search for and select the free Free Bombs package on. Until they add this, I'm afraid you'll have to search and ask for each asset. Open the Unity Asset Store in a web browser and log into your Unity account. It's unfortunate that the Asset Store page does not provide a clear 'license' section. I'm not exactly sure what license apply here, so I'll suggest you ask the author what are the terms of use of their asset before you commit to it. This includes 50 Free models, Programming Assets, Materials, Textures, Particles. If you get the SciFi Robots, you'll find no license on the Asset Store page, and no readme and no license.txt in the package. A selection of the BEST FREE assets for Unity, for December / January 2022.

If you get the Standard Assets provided by unity, you'll have to dig, find, read and understand the actual license, where there is no mention that you must credit the author

Explore user favorites like Massive Crosshair Bundle, Easy Ghost Shader, and Realistic 3D Fireworks Effects. The best place to find Unity Asset Store assets that usually cost money but are free (at least on the day they are posted). If you get the 'Unity-Chan!' Model, you'll have to display somewhere the 'license logo or the license sign separately' as specified by the “Unity-Chan” License Terms and Conditions Discover 10 Free Unity Assets for Real-Time Games, Animation, Architecture, and More. Packed with more than 100 feedbacks, it’ll let you easily trigger screenshakes, animate transforms, play with sounds, cameras, particles, physics, post processing, text, shaders, time, UI, and so much more. I'm assuming that by 'free asset', you mean an asset available on the unity Asset Store that is shown when filtering by 'free' ( resulting in something like this). Feel is the best way to improve your game’s feel and make it extra juicy.

Unity asset store free assets