There is one list Clients, and another list Projects. Latter is linked to Clients by looking up Clients ID column.
Now if I am in Clients list page, how can I click one client row and somehow display all the linked projects rows?
In SharePoint 2013 and earlier you could have used a URL parameter to filter the Projects list web part by the ID of the Client, but currently, Modern SharePoint does not have the web parts or functionality to do that.
You could use an embedded PowerApp, though. Use the Client as one data source, use a drop-down to select the client, then use the Projects as another data source, filter by the selected client ID and show the results in a gallery.
You can create edit/view form of 2 related lists, reference: