SharePoint Online Modern Page Calculated Choice Column in Document Library

Mark182 

Create one Country List in which we are storing our Country values in Title field.

We have another list named States, in which we are storing state values in Title field. This list also has a lookup field named Country in which we are storing corresponding countries from Country List.

States List with Country as Lookup field. Now we have third list named Information in which there are two lookup fields: Country and State

Just add below script in Content editor webpart on NewItem and EditItem page.

<script src="//code.jquery.com/jquery-1.10.1.min.js"></script>

<script type="text/javascript">
$(document).ready(function() {
$().SPServices.SPCascadeDropdowns({
relationshipList: "States",
relationshipListParentColumn: "Country",
relationshipListChildColumn: "Title",
parentColumn: "Country",
childColumn: "State",
debug: true
})
})
</script>

Please let me know if this helps.