Unable to ping DNS any external IP address after reformat and fresh installation of macOS Catalina
I’ve been in your shoes and it’s so hard to go back to the basics.  If a computer can open the web page of the local router or another computer on the internal network - fix your router (after powering it down completely and all network gear to get a fresh start) or find a new network to test is my advice.

If you can ping a router but not past the router - that's a router problem.
If you can reach http on the router or locally but not past the router, that's a router problem.
Networking is fiddly and hard - try to use DHCP and flat settings and fix the network so client setup is automatic and easy.

But, since we love to test and fix - here's my playbook for working with cranky networks. (Or worse, machines all in odd states from people that were well intentioned, but ended up breaking the network settings or diverging from what is needed on a specific network).

Make a new admin account and delete everything network related. All interfaces, all adapters, all settings. Put nothing in the keychain of the new account and delete all other accounts on the Mac.
Set everything to DHCP or connect to a cell phone WiFi hotspot.
Open Safari and browse to captive.apple.com
Focus on http and not ping - ICMP traffic is heavily filtered and dropped from some networks.
Don’t try to overcome the first error you get, but learn why it failed. You shouldn't need to override DNS or bypass the router - the problem is your router, not the OS.

Pretty much the only thing I would do is issue a reachable command from the command line.
scutil -r www.apple.com
scutil -r www.google.com

If the web doesn't work to basic properties, get a network that is appropriate for testing - you wouldn't have to carry all that water you tried to debug a new OS setup - something is very off on the network to need all that.