The most obvious difference between zero- and first-party data is source. Zero-party data comes directly, intentionally and voluntarily from the user. They know it is being collected because they are providing it — typically for some benefit to themselves — and for a specific purpose. This generally means the data is high quality and doesn’t require an additional request for consent, since the company simply wouldn’t have it if the user didn’t want them to. It can be useful and valuable without requiring aggregation and analysis. It enables the company collecting it to achieve specific purposes or goals.
First-party data collection more significantly benefits entities that are not the consumer. The data is collected using online technologies by the company that wants to use it. So this data is indirectly obtained. Without notification required by regulations, it is likely the consumer would not know it was being collected or why.