@ares ,
Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
Generally, Azure advertises the routes according to the VNET Ranges.
I am not aware of any mechanism available to summarize the advertised prefixes in ExpressRoute.
You can use Get-AzExpressRouteCircuitRouteTable , to get a list of all the routes advertised and learned.
Hope this helps.
Cheers,
Kapil