Is true that u can access CT only after u completed the main game's plot?
We don't know when it will be accessible: they haven't officially confirmed it. We'll have to see when the Crown Tundra actually comes into play and we can actually play it, and perhaps when the levels get datamined.
I imagine at least some of it will be post-game at least, but the entire segment being post-game wouldn't be out of the question. Early-game is most certainly not in the cards, however.
The earliest I could conceivably see CT being accessible is around mid-game, around the time the player reaches Hammerlocke after getting three badges. If you look at the map of Galar there's an unused train station between Motostoke and Hammerlocke, closer to Hammerlocke, which could conceivably be the Crown Station where people take Corviknight taxis to the Crown Tundra. The Isle of Armor itself is designed with the early game in mind, with the idea being you start it around the time you sign up for the Gym Challenge, and intertwine the trials with the first three badges (the campaign levels pre-Champion Cup are around Level 10 for the first Avery/Klara battle up to Level 30 for the towers). Considering Urshifu can be obtained as low as around Level 30, it wouldn't surprise me if we could obtain the Legendary Birds and the Regis pre-Champion Cup at around Level 35 for the former and possibly Level 40-ish for the latter, considering the old birds and Regis have tended to be obtainable pre-Champion Cup since they're minor legendaries. I think it's conceivable that *at least* the Galarian Birds and Regis could be obtainable during the main game's plot due to how relatively minor they are, but I definitely imagine Calyrex and other returning raid legendaries and such will most
definitely be post-game, especially since from IoA, the G-Max Urshifu miniquest and the final battle with Mustard are post-game only.
If it does end up being a mid-game entry, I imagine the idea will be you intertwine Isle of Armor with the first three gyms, "finish" it (ie up to the Towers of Two Fists and evolve Kubfu), then intertwine Crown Tundra for the rest of the game up to the Champion Cup, then circle back to both IoA and CT to finish them up for the post-game. I imagine CT will definitely also have some of its areas be post-game oriented, since some of the wild Pokemon from datamined encounter tables and recent screenshots provided by the website suggest there can be wild encounters ranging from Level 60-63. Of course, that could also mean *everything* in the CT is post-game, which is the other possibility. We'll have to see about that when it actually releases, however.