We never had this in the Netherlands. But we got them later.
In fact for a long time they were the only way to be reachable in hospitals because they banned mobile phones for fear of interference with medical equipment.
No they don't interfere because they only receive. They send nothing. At least the pagers of those days didn't, two way pagers did exist but they were super expensive and really uncommon.
To be honest I wish I could still buy one because I'd love to be reachable but not trackable at times. Unfortunately Spain has no pager network in operation anymore.
In regards to how they work, the messages were broadcast over the whole country. Because the network had no idea where you were or if you even were reachable. This is why they had to be so short and why text paging was expensive.
The messages were not encrypted either so everyone could see them. It was not a perfect system but really most people just received codes or at most a phone number.
There were two way pagers (receive and send). The receive only ones rarely interfered with medical equipment. But occasionally the local oscillators on receive only pagers would interfere with medical or sensitive research equipment.
We never had this in the Netherlands. But we got them later.
In fact for a long time they were the only way to be reachable in hospitals because they banned mobile phones for fear of interference with medical equipment.