Infinity Square/Space. The prototype of the game is open source.

This prototype of the game is completely playable, but nevertheless, this is not a complete game.
The source contains all the tools for the development of the game, but you need the above-average programming skill.
Features: infinite procedurally generated world, almost complete destructible of everything, a very large number of NPCs (up to 1000 in one star system), battles involving hundreds of NPC, gravity is an important game element.

https://nvjob.github.io/unity/infinity-square-space