如何确保在每个操作系统和设备上显示所选字体？

According to my understanding every OS has its supported fonts. If a site is using another font that is not supported by an operating system, then a default font will be shown, like on Android the roboto. At the same time I have come across sites that are using a font that is clearly not on the iOS font list, yet on all devices including iphones and ipads and all other operating systems it is displayed right. So how is that possible? Is there a solution around this problem? Thanks,According to my understanding every OS has its