Link to mod: https://mods.factorio.com/mod/Repair_Turret
They are unlocked with green science.
There are some technologies that make the turrets more powerful and more efficient, so in the end game they can hold their own against behemoth biters.
Also they act as construction range extenders, so you don't need to build roboports along all your walls, using repair turrets will be enough to let bots rebuild destroyed entities. You will need to build roboports to house the robots, and provide the logistic area for the chests.
After all upgrades, they can even hold the line against behemoths: https://gfycat.com/widedesertedaltiplanochinchillamouse