News of the Project Fi expansion was first reported by BGR, which apparently blew Google's embargo and then quickly pulled its article offline. However, eagle eyes were able to glean some details including the fact that iPhone support will reportedly be in "beta" -- at least initially.
Although we don't know exactly what beta experience entails for iPhone users, it stands to reason that some features may not be fully implemented given Apple's tight control over iOS. With that being said, as it currently stands, Project Fi only supports recent Pixel 2/Pixel 3 smartphones, and a handful of smartphones from LG and Motorola.
If you aren't familiar with Project Fi, it defaults to Wi-Fi calling when available, and can automatically switch between Sprint, T-Mobile, and US Cellular networks when you're out and about based on cellular reception. Customers pay a base rate of $20 for unlimited calling and texts, while data can be added at a rate of $10/gigabyte.
With Bill Protection, your monthly outlay is capped at maximum outlay of $80 per month ($20 base rate + $60). However, if you go over 15GB, you'll see your download speeds throttled.