Cdijk21's answer is good, but it's lacking a few points, which I'll try to cover in my answer.
Defensive typing
SS OU is a metagame which can't get enough of Kartana and Rillaboom. While the latter has, of course, fallen a bit off, it still poses a threat to a lot of offensive builds. The catch here is the common Grass-typing. Zapdos, with Electric-/Flying-, resists Grass- and can easily counter these Pokémon. Other than the ubiquitous Grass-types, Zapdos also fares well versus other offensive threats in Urshifu-RS and Ladorus-Therian, as well as defensive chords in Tornadus-Therian and Corviknight, etc. Zapdos can also block U-Turn from most Pokémon in the metagame.
Although useful, Zapdos' defensive typing isn't the 'best', on the whole. It comes with a disclaimer which explicitly states that you'll have issues against certain Pokémon that you're supposed to check. It cannot absorb Knock Off from Pokémon like Weavile, Rillaboom and Kartana, as chipping it is comparatively easier without its item. Corviknight and Skarmory arguably have better typings, as they're not weak to Stealth Rock and can handle their item being Knocked off.
Ability
Static and Pressure are both good abilities, in their way. Static punishes U-Turn users hard, as they usually wouldn't appreciate a paralysis. Pokémon such as Weavile can't freely use Knock Off, if they anticipate a predicted Zapdos switch-in, as getting paralysed would hinder them severely. Knock Off affects its ability to consistently check the aforementioned Pokémon, so Zapdos often relies on Static to punish offensive threats.
Pressure isn't a bad choice, either. Substitute + Pressure, which is mostly seen on Kyurem, is another viable defensive Zapdos set. It aims at PP stalling most passive Pokémon such as Blissey, Heatran and Toxapex, as they can't harm Zapdos directly through non-Status moves. This is usually outclassed but is a niche option and almost everything with a small niche works in OU.
Hazard control
- Zapdos forces a lot of switches, especially versus the offensive Pokémon mentioned above. This lets Zapdos use Defog freely, thus adding to its utility. Although Zapdos can't use Defog as efficiently as it did in the previous gens, it's still a great hazard control Pokémon currently. Against non-Toxic Landorus-T and Clefable (and Hippodon, though it's too niche), it fares pretty well and can win hazard wars versus them.
Flexible moves
With access to great utility moves in Toxic, U-Turn and Defog, as well as reliable recovery in Roost, Zapdos' moveslots are usually extremely flexible and versatile. Toxic, although not used much, is a great option to put threats such as Landorus on a death timer, thus decreasing their overall utility and feasibility. U-Turn is a choice over Volt Switch, to pivot against Ground-type foes as well, which comes extremely useful.
Discharge can spread paralysis, which makes it difficult for its checks such as Heatran and Slowking-Galar to switch freely into Zapdos. It's also a great STAB option and, in tandem with Heat Wave, keeps most of the Pokémon Zapds are supposed to check at bay. Heat Wave hits Steel-types such as Ferrothorn, Skarmory and Corviknight, thus enabling Zapdos to perform its duty as a hazard controller as well.
Hurricane is a great option to severely punish Ground-types such as Garchomp and Landorus-T. The offensive sets of the former are hit especially hard as they're usually not invested in special defence. Hurricane + Volt Switch is a great choice, as with Volt Switch, Zapdos can maintain momentum and dent the Ground-types which block Volt Switch with Hurricane. This comes at the cost of not being able to hit Steel-types for a lot, though, which can pose to be an issue long-term.
Teambuilding
Building around defensive Zapdos is exceptionally easy, as it doesn't require much conditioning. Pokémon which can take care of Ice-types like Weavile are great partners. Scizor fulfils this role exceptionally, with the two benefitting each other mutually, and also forms a VoltTurn core with Zapdos. Landorus-Therian is also a good choice as it can check Zeraora and Garchomp easily, which can muscle past Zapdos late-game. Knock Off absorbers like Clefable are also appreciated.
Offensively, Kartana and Rillaboom are eminent partners. They take care of Ground-types relatively easily, and Zapdos can eliminate pesky Steel-types and Flying-types which lets the aforementioned spam their Grass-type STABs with ease. Melmetal is also a good niche partner as Zapdos can pivot into Melmetal easily, thus making it easier for Melmetal to not take chip damage on the switch-ins. Dragapult and Zapdos also complement each other's ability to take care of their checks and counters.
On the whole, Zapdos is a consistent and great wall for SS OU. It's been a prominent threat in OU in every generation since it dropped, and has a lot to its merit. For more on Zapdos, check this and this out.
