Games like PROJECT R1
Games like PROJECT R1 include Kerbal Space Program, while True: learn(), Garry's Mod, PC Building Simulator, and Poly Bridge. We matched them by the tags players give PROJECT R1 on Steam (Hobby Sim, Education, Sandbox, Simulation, and Building): closest matches first, and among equally close ones the better-known games.

Kerbal Space Program
Shared: Sandbox · Simulation · Education

while True: learn()
Shared: Education · Simulation · Programming

Garry's Mod
Shared: Sandbox · Building · Physics

PC Building Simulator
Shared: Education · Simulation · Building

Poly Bridge
Shared: Building · Simulation · Sandbox

Space Engineers
Shared: Sandbox · Building · Simulation

People Playground
Shared: Sandbox · Physics · Simulation

House Flipper
Shared: Simulation · Building · Sandbox

Totally Accurate Battle Simulator
Shared: Sandbox · Simulation · Physics

Besiege
Shared: Sandbox · Simulation · Building

Universe Sandbox
Shared: Sandbox · Simulation · Education

Stormworks: Build and Rescue
Shared: Sandbox · Building · Physics

Poly Bridge 2
Shared: Building · Simulation · Sandbox

Kerbal Space Program 2
Shared: Sandbox · Building · Simulation

Automation - The Car Company Tycoon Game
Shared: Simulation · Sandbox · Building

Universe Sandbox
Shared: Sandbox · Simulation · Education

Car Mechanic Simulator 2018
Shared: Simulation · Building · Sandbox

Aviassembly
Shared: Building · Sandbox · Physics

Juno: New Origins
Shared: Building · Education · Programming

Fun with Ragdolls: The Game
Shared: Sandbox · Simulation · Physics

Cities: Skylines
Shared: Simulation · Building · Sandbox

Scrap Mechanic
Shared: Sandbox · Building · Physics

Main Assembly
Shared: Sandbox · Building · Simulation

Flyout
Shared: Sandbox · Simulation · Building