As a pay-to-play game, you first have to buy the base game of The Elder Scrolls Online to get access to the online servers. After that you can play the entire game. Optionally, you can purchase DLCs, a subscription and items from the in-game shop.
With the exception of clear group content such as multiplayer dungeons and raids, you can play all of The Elder Scrolls Online’s content alone. The quests, which are fully voiced are particularly easy to enjoy alone, especially if you like to read and listen to everything at your leisure. The difficulty of the opponents will adapt to the level of your character. It all depends on your fighting skills and of course your equipment.
The subscription is especially valuable for players who want to intensively use the crafting system or have already played through all the quest content of the base game: Collected crafting resources are automatically put into an extra crafting bag instead of filling up the inventory with its limited slots. As a subscriber, you will also receive a 10% bonus on experience and crafting experience gained, access to all DLCs except the larger chapters (Morrowind, Summerset, Elsweyr, Greymoor), and a certain amount of the premium currency Crowns for the in-game shop (1,650 Crowns for a one-month subscription). So if you want to play TESO more intensively or are curious about other areas, a subscription is worth a look.
Bethesda regularly provides ESO with new updates. Currently the game is updated four times a year with two dungeon DLCs (at least two new dungeons per DLC), a new, extensive story chapter and another smaller region DLC. Since 2019, all content published each year has been contextualized. The associated story evolves from the first to the last new content of the year. All information about the Elder Scrolls Online Addons can be found in the linked article.
Crossplay at The Elder Scrolls Online is not yet possible, the players of the different platforms PC and console must stay among themselves. Creative Director Rich Lambert said at E3 2019 that crossplay is not planned as a feature for The Elder Scrolls Online.