Query WMS for available layers using QGIS 3

There is no attribute table for a WMS layer available in a client (QGIS or ArcGIS). A query to a WMS layer is a query to the server providing the map (a GetFeatureInfo request) querying a point location (a pixel in the map image). If the layer is queryable the server sends back a response for the attributes at that point in the underlying data. Whether a layer is queryable in this way is discoverable from the GetCapabilities request to the service, through the queryable attribute on the layer.

Below layer is not queryable:

<Layer queryable="0" opaque="0" cascaded="0">
    <Name>BGS_625k_BAR</Name>
    <Title>GBR BGS 1:625k Bedrock Age</Title>
    <Abstract>GBR BGS 1:625k Bedrock Age</Abstract>
    <KeywordList>...

Below layer is queryable:

<Layer queryable="1" opaque="0" cascaded="0">
    <Name>GBR_BGS_625k_BLT</Name>
    <Title>GBR BGS 1:625k Bedrock Lithology</Title>
    <Abstract>GBR BGS 1:625k scale Bedrock Lithology</Abstract>
    <KeywordList>...