Type: Null and consequently Silvally can be obtained at Wyndon Tower after beating the game.
Eternatus is obtained through the main story.
Zamazenta can be obtained at the Slumbering Weald in the postgame.
If you initiate a trade with someone that has the DLC via local communication, you can get every other legendary in the game, except perhaps Zacian if they have Shield, if they're willing to trade.
If you have subscriptions to Bank and HOME, you can bring in legendaries from older games.
Hope I helped!