Please recommend it as a second mobile phone

iPhone 6s is pretty okay. It'll get you from a to b. If you're buying from someone, I'd hold off since it's likely too old. The battery will likely be bad. As for handling common apps like facebook and youtube, it'll do the trick. I wouldn't expect to be able to play any games though since its OS is outdated. I'd recommend google's pixel 3a. It's cheap and powerful. Also google pixel 3 is 300 bucks off until the end of september.