Thank you so much for doing the homework task. I appreciate your effort.

As described, the easiest way is always Coinmarketcap or Coingecko to find the list of explorer(s) for any Blockchain. The user just need to search the coin name in Coinmarketcap and then look for the list of explorers by clicking on Explorer.

Sending and receiving is one of the main building blocks of blockchain. To send and receive bitcoin, you need to have a wallet where you need to put the public address of the sender and recipient.

Sending and Receiving are the financial use case of a Blockchain, they are not the building Blocks.

The Building Blocks are- root of trust(genesis block), immutability(can't be erased), an ordered Blockchain(each block contains the hash of previous block & timestamp), append-only( you can only add a block, can't delete a Block), etc.

Each wallet is having a key pair- public key & private key. A public key is derived from the private key and the public key, in reality, is lengthy and complex, to make it simple and easily usable for the common man, it is hashed into a simpler form and thus the wallet address is derived from public key using a suitable hashing algorithm (the hashing algorithm may vary from one blockchain to the other).

A blockchain address is a unique sequence of numbers and letters and functions very much like an email address. It refers to a specific destination on the network where cryptocurrency can be sent to, and it can be used only once

You are right, but it can be used for more than once. For security reasons some people use it once, but you may continue using it multiple times, it is up to you.

That means, the address is valid.

You got it right. A valid address will always reflect in the explorer when you query, whereas an invalid address won't.

Thank you so much.