The membership works like it would if you were to join a library. You have the stuff on loan to you as long as you pay the membership fee. If you stop, you lose access to your content at the end of the subscription period. However, should you decide to join in again at a later date, all your stuff that you got with your subscription is available again. This has its advantages and disadvantages. The advantages are you get the latest of everything, get all the stuff you would normally pay for outright, and if you're one that doesn't look at Trainz for months at a time, due to a massive honey do list, traveling, or just not wanting to use Trainz, you can come and go as you please and everything is there when you come back.
The disadvantage is it's a monthly fee depending upon which level. For those at the highest level, it's about $14.00 per month US.
For those that just bought the program outright, there is no membership required. And it's just like the olden days where they have to purchase all the extra downloadable content, found in the Trainz Store. The program and content works all the time, it does require a login to the internet once a month to validate the ownership, and there are no frequent program updates. The program updates come at the regular release schedule such as service packs, hot-fixes, and new versions.
The DLC, or Downloadable Content, is stuff made for sale such as routes, locomotives, sessions, passenger, and freight consists, etc. These generally are of very high quality, such as the locomotives and routes from Jointed Rail, or the Canadian routes from RoysTrainz.
The DLS is the free stuff made available from the community at large. The quality varies, however, there is some truly great stuff up there such as buildings by David Snow (Dave Snow), and various assets by MSGSAPPER, and so many, many others.
Hope this helps 'splain this.
As far as installing the routes from TS12 and TS2010. It can be done, however, there maybe issues with the routes from TS12. If you have installed TS12 SP1, you can't export the routes or any built-in content due to the assets being locked. If you attempt to extract them, they are faulty and can't be repaired at all. TS12 prior to SP1 will work, and so will route from TS2010. If you do bring the routes into TRS2019, be prepared for downloading assets, and asset updates from the DLS.