purple health on destroyed buildings?

[Dr. Eggman]

bout what the title asks

[Steelwind]

Pretty sure that is the timer when the ghost expires. The ghosts start showing up when you research bots so they can replace destroyed structures.

[Dr. Eggman]

do they automatically do that when you have the items to replace them or do you have to set them to replace specific types of buildings?

[Fel]

The construction robots will try to rebuild them if they have the right items.

Basically, think of it as the same thing as the ghosts from blueprints, the only difference is that they will be removed after a while if they are not rebuilt (in case you don't want to rebuild for example).

[Steelflame]

If its within their range (which is pretty freaking huge, you can generally cover most bases for construction bot purposes with 1 or 2 of the roboports tops. Generally because you need to keep stuff in range of the much smaller Logistics port ranges anyway you'll always have everything covered for the construction bots) they automatically will do it so long as they have materials in the logistics system (Or your inventory if they are specificly coming out of a personal roboport) where they can access them.

Ideally, you would want to make a provider chest that just has an input of a few stacks of spare walls from an auto production setup that keeps it topped off, as well as a few others for your gun/laser turrets and perhaps gates, although you probably don't want to automate gate production and just keep a fairly large stack ready to go instead.