https://assetstore.unity.com/packages/3d/environments/simplepoly-city-low-poly-assets-58899#description

https://assetstore.unity.com/packages/2d/textures-materials/sky/fantasy-skybox-free-18353

→ see github for list as well

• Fantasy Skybox FREE by Render Knight https://assetstore.unity.com/packages/2d/textures-materials/sky/fantasy-skybox-free-18353
• SimplePoly City - Low Poly Assets by VenCreations https://assetstore.unity.com/packages/3d/environments/simplepoly-city-low-poly-assets-58899
• Deep Space Skybox pack by Sean Duffy https://assetstore.unity.com/packages/2d/textures-materials/deep-space-skybox-pack-11056 Free Demo Of Low Poly Space Alien Worlds 3D Asset Pack by Draftpunk Studios https://assetstore.unity.com/packages/3d/environments/sci-fi/free-demo-of-low-poly-space-alien-worlds-3d-asset-pack-258683

Sounds:

• Running, Tennis, Hitting-balls sound effect by freesound_community https://pixabay.com/sound-effects/film-special-effects-tennishitting-ballsrunningbirdsmono-22566/
• Jogging Past Footsteps Back and Forth in Tennis Shoes on Sidewalk by freesound_community https://pixabay.com/sound-effects/film-special-effects-jogging-past-footsteps-back-and-forth-in-tennis-shoes-on-sidewalk-28098/
• 43 tennis by freesound_community http://pixabay.com/sound-effects/film-special-effects-43-tennis-57477/
• Space by freesound_community https://pixabay.com/sound-effects/musical-space-72679/
• Clock beat effect by H-beats https://pixabay.com/sound-effects/musical-clock-beat-effect-402705/
• Cinematic Rumble by freesound_community https://pixabay.com/sound-effects/film-special-effects-cinematic-rumble-106063/
• Game Gaming Minecraft Background Music by BackgroundMusicForVideos https://pixabay.com/music/upbeat-game-gaming-minecraft-background-music-379533/
• Space trip by PlaySound https://pixabay.com/music/build-up-scenes-space-trip-114102/
• Fast Chiptune for Gaming Videos by nickpanek https://pixabay.com/music/video-games-fast-chiptune-for-gaming-videos-253097/
• Deep and Mysterious Music for Cosmic Themes by ViraMiller https://pixabay.com/music/ambient-deep-and-mysterious-music-for-cosmic-themes-273579/
• Deep House - Low Gravity by AntipodeanWriter https://pixabay.com/music/deep-house-deep-house-low-gravity-13045/
• Success Videogame SFX by Mrstokes302 https://pixabay.com/sound-effects/success-videogame-sfx-423626/
• Verloren by KastenFrosch (freesound_community) https://pixabay.com/sound-effects/musical-verloren-89595/