>What do they do with the heat in summer?
They use to heat the hot water for the house (taps, showers etc.). District heating systems heat both the radiators for heating the house, as well as for the domestic hot water supply.
Still true, during the summer there is probably an excess of heat that is unused and just vented. Of course better to vent only some of the excess heat in the summer, that all of the excess heat all year round.
A lot of other posts talk about water cooling the server equipment, which is a more efficient way to capture waste heat. However I seriously doubt that is the case here.
More like they use a waste heat recovery system that captures the heat from the outgoing air that was used to cool the servers.
Water based server systems require a lot of plumbing, pumps, heat exchangers and drains in case of leaks. It also requires specialized server hardware, which although it exists, is not standard and not ideal for a data center which has a short life cycle and high turn over of sever hardware for new better models continuously.
An air based waste heat recovery system is much easier to install and is independent of what server hardware you use.
The main reason this is not done more wide scale, is it is rare you put a huge data center into an urban area, that is also equipped with district heating. Urban land is generally very expensive and so it is much more cost effective to put your data center in a remote location, far away from urban areas and far away from district heating systems.