The default attribution model used is that Matomo attributes goal conversions to the Last Non-Direct channel seen by visitors (their referrer).
The Last Non-Direct channel
In the context of web analytics, the Last Non-Direct channel refers to the last marketing channel, excluding the Direct channel, through which a user interacted with a website before completing a conversion. In other words, it shows the last source that contributed to the conversion, excluding instances where a user directly typed the website URL into their browser or clicked on a bookmark to access the site.
Change the default to “First Interaction”
You can customise the default channel for each website, as explained in this FAQ: credit conversions to the first referrer instead