Let’s say for example, I am at my friend’s house, and for one reason or another, I don’t have my phone.

If you need to log into your friend’s laptop to check your email, you would need your phone or some other passkey you had set up for your account, yes, as long as that was the only login method you have setup on your account. If you don’t have your phone, you might not be able to pass the two-factor steps or account login location checks many accounts. If Google finds the new login attempt suspicious for some reason, it will ask for additional checks like a code sent to your email or through a text and you may not be able to log in with just the password anyways. Just because you have the right username and password, it doesn’t mean that a service may let you log in without access to some kind of other trusted information accessible on an existing device.

Overall though, think of it like forgetting your physical keys.

Does that also mean that if I forget to log out of passkey, they can access all of my accounts correlated with my passkey account?

Yes, the same as if you had left your physical keys there and those keys provided access to all your accounts. There may be some technical protections like the timeout until it locks on a password manager but that’s up to the password/passkey manager app to implement and for the OS to guarantee the security of. It’s no different from loading up your password manager on the device. If you don’t trust the device or the owner of the device, you should not access your password/passkey manager on it.

what happens if my passkey account is compromised? All of my accounts are linked to a single point of failure?

The same thing that happens if your password manager is compromised: you secure it (rotate encryption, create a new database, however you want) and then you set about updating new passwords and passkeys for your accounts. That’s why it’s recommended to only have your actual password/passkey manager on something you trust (your phone, your computer, etc) and use that device as the passkey for whichever other device your logging into rather than loading up your password/passkey manager on each device you’re logging into.

A friend of mine had to break out some kind of USB dongle to log into his Google account on a new machine the other day. Is that a form of passkey?

It’s a form of WebAuthn credential most likely, yes. Passkeys aren’t actually entirely new in how they can be used with accounts, the standards have been there for a while now. It’s mainly just a unified marketing from the big players as well as developing an ecosystem around it the standard such as the protocols for using a phone via Bluetooth as a passkey on a desktop/laptop to log in and other things like syncing the passkeys between devices using their existing password manager services for user convenience (so that the average person can actually use them). Under the hood it’s still WebAuthn for the actual authentication. Hardware security keys that connect via USB, Bluetooth, or NFC have been around for a while but have usually operated in nonresident key mode where they’ve been used for second factor authentication. Nonresident key mode has the advantage of storing the private key in an encrypted format with the website or service your logging into, meaning that the actual hardware key doesn’t need to have any storage capacity and can work with an infinite number of sites. This has the disadvantage that you have to provide a username (and typically a first factor like a password) to lookup which keys should be used (ie the ones associated with a specific account). That is probably how your friend logged in with a USB dongle. WebAuthn credentials that operate in resident key mode like passkeys do on the other hand store both the information related to identity and authentication, meaning that all you have to do is select the account you want to log into. This requires that they are stored on a trusted device like a phone, a laptop, or a hardware security key dongle that has storage.

What happens if that dongle gets lost/stolen/broken? Or what if you just forgot it at home? Are you SOL?

Again, the same thing that happens when you forget your physical keys for your car or home. You can’t access the thing protected by them until you go get them. The alternative is to bypass the normal authentication workflow and work around it, such as with an account recovery process (similar to getting a locksmith to get back into your car or home).

I am all for more security and less password remembering, but I hop around a lot of computers.

Then you’d probably like being able to log in by just unlocking your phone and confirming things, rather than having to go through a password lookup and one time code entering process each time.