Besides Latios/Latias, your cover Primal (Groudon and Kyogre), Rayquaza and Deoxys, who you’ll all meet during the course of the main story, there are a bucketload of other Legendary Pokemon to catch, most of which are available from Mirage Spots.
The capturing techniques for each are much the same. For the most part, you’ll want to status them - preferably with Paralysis - and then use moves like False Swipe or Hold Back to slowly grind down their HP to just 1. (It will take a long tme for Regirock and Registeel, but it’s better than making them faint.)
From there, mostly just toss out Ultra Balls, or preferably Dusk Balls when it’s night time. Though, even at 1 HP and ailed, it will probably take a while to snag the Legendary. If the battle does begin to drag, switch over to Timer Balls instead. Basically, be sure to bring lots of items!
Dusk Balls can be purchased from the Fallarbor Town Poke Mart. Meanwhile Timer Balls are sold from the Rustboro City Poke Mart, but you must speak to the Scientist outside Verdanturf Tunnel, from the Rustboro side.