Thank you so much for taking on the homework task. You have very nicely detailed out with examples. I appreciate it.

I just need to say you that the easiest way to find an explorer of any Blockchain is through Coinmarketcap/Coingecko. Google search may not be that precise.

Examples? (How to find such block explorers?)
In google search, you can make the query "blockchainname block explorer". That means when you make use of any other blockchain then you can search for the name. e.g. Ether blockchain can be queried using the query. "Ethereum block explorer". Like this you can find blockchain explorer for any blockchain on Google.

I think the best and the easiest way to find a Block explorer of any Blockchain is to visit Coinmarketcap or Coingecko. There you can easily find out the list explorer available for that chain. You can use any of them.

For example,

• Go to Coinmarketcap.com
• Search any coin, say XRP
• Then click on Explorers

How do I check whether a wallet address is valid for a network or not(using Block explorer)? Examples?

In the search box of any Explorer, directly enter the address of that Blockchain. If it's a valid address then it will show you the details of the address -- balances, number of transactions, etc. If it's an invalid address, then it will say "we can't find it".