This is what the user who pays gets access too. Sometimes the full version is obtained by entering some sort of password into the demo version. Sometimes the full version is mailed to the user on disk. There's a variety of different ways to get the full version in the hands of the user. Once the two versions are ready, the demo version is distributed as widely as possible.
Users find it and try it out.
If they like it, they pay for the full version, which is promptly sent to them. This is how the system works, more or less. All a person needs to release a shareware product is a computer, some software, and a lot of time. Advertising is cheap on the net and no products are shipped in boxes, so costs are extremely low. The profit margin for shareware is exceptionally good.