These are the Yunnan icons for iOS. They follow the iOS Human Interface Guidelines (Apple could have done it better though), first introduced in iOS 7 and supported in all later releases up until now (at least iOS 11). These are outline icons; they're based on thin two-pixel lines and is optimized for 50×50 pixels. If you're creating an iOS app, either download Yunnan PNG icons in 3 sizes (free icons for 50 and 100px; for folks who paid there's 150px) or download a single vector icon in PDF format (you'll have to buy icons) — Xcode will generate the proper PNGs on the compilation. There's a matching set of glyph icons — Apple introduced them in iOS 11 and uses for tab bars.