Group messaging improved in iOS 14 and iPadOS 14, but two of the best features are hard to discover by accident: replying just to one person within the group, and mentioning one or more people to highlight that you want them to see a message.
In a group chat in Messages, press and hold on a message, and the usual array of reply options appears, including reactions. Tap Reply, and the message appears in a sort of isolated environment, in which the rest of the group’s messages are visually blurred out and you just see the person and their message to which you’re replying. You can type in a reply, and send it. (To exit that mode without replying, just tap elsewhere on the screen above the message.)