They all do that. I've been in Offensive Security for 10+ years with several spent at FAANGS, and not only do they all have large security teams doing internal testing, they hire multiple contractors like Trail-of-Bits to audit every important service continuously throughout the year.

Apple has way more than 10 full time researchers looking at iOS all day, trust me :). They also have a really generous bug bounty. There is always bugs though.