The AffectedAssetType is available as extended properties. For your query, you can try something like this.
SecurityAlert | where TimeGenerated >= ago(1d) // Adjust the time range as needed | extend AffectedAssetType = tostring(parse_json(ExtendedProperties)['AffectedAssetType']) | summarize AlertCount = count() by AffectedAssetType
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.