So, i bought a new pc and i am thinking of buying Train Simulator, but i don´t really like the routes that come with TS2021, so can i instead buy TS2019 which i prefer due to the routes that come with it, and then will the game update to the TS 2021 for free right??
Correct. You'll need to source a key for TS2019 from somewhere, but as soon as it's activated on Steam you'll get TS2021 with the 3 routes that come with TS2019, but not the routes in TS2021.
Assuming you can get hold of Steam keys for them you can buy any number of old editions. If you already have TS the routes get added to your collection as if you had bought them separately. It's a good way to accumulate a cosmopolitan collection of routes usually for a few pounds per key and in at least one instance it's the only way to get a route now unavailable otherwise. As you have noted an old edition may have a bundle of routes that interests you more than the current offering- I'm with you there actually, someone was kind enough to give me a key for TS2021 and it's a bit underwhelming. TS2019 was a good bundle. You have other options to get started. If you look through the Store you'll find some routes don't require you to have TS- they're effectively single route editions. Faversham High Speed and West Somerset Railway off the top of my head, there may be others I've overlooked, as well as the novelty routes Game of Gnomes, Holiday Express and The Count of Monster Disco. If you are interested in British trains there's the Great British Railway Journeys tie in with five routes. Having TS does not preclude you from buying these in the future- ideally when there's a sale on.
I had a look at that package "Great British Railway Journeys" and it´s really interesting. Do i need to have TS to purchase that??
If you look at its Store page you'll see two options- one you can start with and the "complete my collection" version which is what you want if you already have TS. Note that one of the routes is the newer Portsmouth Direct which comes with TS2019.