Servers with Public IP from ASN

Hey Everyone,

First time poster here but really just want to understand how ISPs, particular web/server providers, provision VPS or dedicated servers with public IPs internally and how the SNAT/DNAT is configured.

We have a /24 ASN with is announced via BGP to our data centre router (Sophos UTM). Currently we have a subnet with a private IP range (192.168.0.0/24) and then DNAT rule from public IP to private internal IP and same with SNAT.

If I wanted to setup a VLAN/Subnet say with a /26 or /28 how can I assign a public IP to a server or VPS etc?

Just trying to understand how the routing works. Do we need a static route that goes to the default gateway of the Sophos UTM?

Thanks!