user5298: I have seen numerous examples of blockchain explorers providing information about the on chain activity of specific dApps. For example:

opencnft.io aggregates data about various token marketplaces
cexplorer.io has a list of the most active contracts and their associated dApps
eutxo.org provides information on transactions associated with a specific dApp

How do all of these sites map smart contracts / script addresses to a specific dApp? Do they manually determine which smart contracts a dApp is using or is this info listed somewhere?

user1142: 
Do they manually determine which smart contracts a dApp is using or is this info listed somewhere?

Yes, they manually gather the data of which script hash belongs to which dApp. This information is not registered anywhere on the blockchain or in any official central place.
There are many ways to gather this information, as suggested in the comments you could use the dApp yourself and check the resulting Tx or simply ask the developers.
If you want to build a new app you could of course also scrape the data from other existing sources like the websites you have listed above.

5298: <p>I have seen numerous examples of blockchain explorers providing information about the on chain activity of specific dApps. For example:</p>
<ul>
<li><a href="https://opencnft.io/" rel="nofollow noreferrer">opencnft.io</a> aggregates data about various token marketplaces</li>
<li><a href="https://cexplorer.io/dapps" rel="nofollow noreferrer">cexplorer.io</a> has a list of the most active contracts and their associated dApps</li>
<li><a href="https://eutxo.org/" rel="nofollow noreferrer">eutxo.org</a> provides information on transactions associated with a specific dApp</li>
</ul>
<p>How do all of these sites map smart contracts / script addresses to a specific dApp? Do they manually determine which smart contracts a dApp is using or is this info listed somewhere?</p>
 1142: <blockquote>
<p>Do they manually determine which smart contracts a dApp is using or is this info listed somewhere?</p>
</blockquote>
<p>Yes, they manually gather the data of which script hash belongs to which dApp. This information is not registered anywhere on the blockchain or in any official central place.</p>
<p>There are many ways to gather this information, as suggested in the comments you could use the dApp yourself and check the resulting Tx or simply ask the developers.</p>
<p>If you want to build a new app you could of course also scrape the data from other existing sources like the websites you have listed above.</p>
