i sent imessage (text) to a friend who has an iphone and the bubblenshows in blue and says delivered; does that mean they got my message?

It does mean that it has been delivered from Apple servers. There are scenarios where you may see delivered and the person has not seen it. What if the person's phone was out of coverage, but they have other devices connected to their Apple ID. In this case it may be delivered to another device and the person without coverage may not have received the message yet. The message bubble would still show blue and say delivered. It does not make a distinction on which device it was delivered to, only that Apple servers were able to send the message to your Apple ID.

They may also have Do Not Disturb on, Screen Time settings restricting notification, or driving a car with focus set to not allow notifications.

The Read receipt is probably a more reliable indicator if the message has been received, but they would need to have that option turned on before you would see their message marked as read.