Games like Under the Ruin
Games like Under the Ruin include Oxygen Not Included, RimWorld, Frostpunk, The Alters, and This War of Mine. We matched them by the tags players give Under the Ruin on Steam (Strategy, Tower Defense, Exploration, Survival, and Management): closest matches first, and among equally close ones the better-known games.

Oxygen Not Included
Shared: Survival · Resource Management · Management

RimWorld
Shared: Strategy · Survival · Management

Frostpunk
Shared: Strategy · Survival · Resource Management

The Alters
Shared: Survival · Strategy · Exploration

This War of Mine
Shared: Survival · Strategy · Resource Management

They Are Billions
Shared: Strategy · Survival · Tower Defense

Frostpunk 2
Shared: Strategy · Survival · Resource Management

Factorio
Shared: Strategy · Resource Management · Survival

IXION
Shared: Strategy · Survival · Resource Management

Banished
Shared: Strategy · Survival · Resource Management

Kenshi
Shared: Survival · Strategy RPG · Strategy

Sheltered
Shared: Survival · Strategy · Management

Endzone - A World Apart
Shared: Strategy · Survival · Management

Cities: Skylines
Shared: Strategy · Management · Resource Management

Settlement Survival
Shared: Survival · Management · Strategy

Plague Inc: Evolved
Shared: Strategy · Tactical · Simulation

Space Haven
Shared: Strategy · Survival · Management

Manor Lords
Shared: Strategy · Management · Resource Management

The Riftbreaker
Shared: Tower Defense · Survival · Exploration

Game Dev Tycoon
Shared: Strategy · Management · Simulation

Plants vs. Zombies GOTY Edition
Shared: Tower Defense · Strategy · Survival

The Wandering Village
Shared: Management · Exploration · Resource Management

Rise to Ruins
Shared: Tower Defense · Strategy · Survival

Dwarf Fortress
Shared: Strategy · Management · Survival