Can I use Coveo to power xConnect?
i.e Use Coveo instead of Solr or Azure for xConnect Search?
I could not find anywhere the guide to use Coveo for xConnect search (instead of Solr).
Right. Coveo is a relevance and intelligent search solution. In the Sitecore ecosystem, it is to be used only for the visitor facing website search components. Even with Coveo for your website search, you still need Solr/Azure search for Sitecore backend search requirements.
Coveo can also be used to index your Sitecore Commerce sellable items for website search usage. However, you still need Solr/Azure search for the Sitecore Experience Commerce features.
Coveo is not a content retrieval index technology like Solr/Azure search are used in the context of Sitecore. You cannot use Coveo to index xConnect, marketting, EXM, social, etc indexes.
Not sure if I understand correctly, but you can't use Coveo to search the xConnect data.
Instead, you can use xConnect to improve the relevance of your Coveo search, which will be done over your Sitecore items.