It is very true that a certain amount of Bitcoin can't be spent, but why is that?
Because Bitcoin works on coins, not on addresses. You have the first coin, which is in this particular output:
https://mempool.space/tx/4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b#vout=0You have transaction 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b and output index 0. If it would be present in the database of all coins, then it would be normally spendable, as all other coins are. However, the Bitcoin client initially started from the empty database. And initially, there was no such thing as "the Genesis Block". You started creating the whole chain from scratch, from zero "previous block hash".
And then, Satoshi created the first block, and hardcoded it in the client. He simply put a rule, that "the block 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f has to be there as the first block". But: he didn't add the transaction 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b to the database! Which means, that the block is there, but the coins are not. And if you create a valid transaction, which will spend the UTXO 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b:0, then it will be rejected by all nodes, because this UTXO is not in the database of all spendable coins.
And what should happen to the other amount on same address?
It can be normally moved anywhere, without any restrictions, as long as you can produce a valid signature. Just because the next coins were created in the next blocks, and are just regular Scripts.
But there haven't been emphasis on if further additions could be made and what will happen to those additions.
They can be normally moved. To test that, you can create a regtest chain, where you can replace Satoshi's address with your own, and just test it. Also, many altcoins can prove you that, for example in Litecoin, you have this address:
https://blockchair.com/litecoin/address/Ler4HNAEfwYhBmGXcFP2Po1NpRUEiK8km2 and as you can see, it is normally spendable in other transactions, except 97ddfbbae6be97fd6cdf3e7ca13232a3afff2353e29badfab7f73011edd4ced9.
As at the time of my writing, this address contains about 100btc with the unspendable 50btc inclusive.
No, because it is P2PK, and it contains only 50,01004040 BTC:
https://mempool.space/address/04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5fHowever, P2PKH version was always fully spendable, and contains 49,99353524 BTC:
https://mempool.space/address/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNacan the remaining btc be spent assuming Satoshi has the private key?
Yes.
If Yes, give your reasons. If No, also share your reasons
Yes, you can test it in your own regtest, or by exploring many altcoins like Litecoin, which copy-pasted the same code, related to the Genesis Block, and used just their own public keys, instead of Satoshi's key.
Are the unspendable coins limited to only the first block (Genesis block) ?
Yes, it is only limited to the coins from the first transaction, which is
https://mempool.space/tx/4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33bIf the first reward (50btc) was intensionally transformed to unspendable coins by Satoshi, does that mean that our current developers or miners can also do thesame ?.
Yes, you can burn your coins by sending them into OP_RETURN. Also, when it comes to miners, they can claim less coins than allowed, and then they are irreversibly burned. For example, this miner burned a single satoshi, and all transaction fees in block 124724 in this transaction:
https://mempool.space/tx/5d80a29be1609db91658b401f85921a86ab4755969729b65257651bb9fd2c10d