The save files are typically located in your Steam user data folder.

Players occasionally encounter "cloud sync" conflicts where the local save differs from the cloud-stored version. It is highly recommended to periodically backup the Shadow of the Tomb Raider folder in your to prevent data loss from crashes or unintended overwrites. Shadow of the Tomb Raider - PCGamingWiki PCGW

: Interacting with and exiting a base camp (campfire) triggers a reliable save point. This provides a manual-adjacent method for players to "lock in" their progress before quitting a session.