Can a unconfirmed transaction be lost or modified? I am beginner and I am still learning fundamentals of block and bitcoin network.
Suppose, I do a transaction on bitcoin network, it is marked as unconfirmed transaction and is stored in mempool of full nodes.
This unconfirmed distributed over network, at some point a minor will create a block which will contain this transaction.
My question is where these unconfirmed transactions are stored before being included in block or hence block chain?
Can they be modified before a minor put them in blockchain?
where these unconfirmed transactions are stored before being included in block

Mempool
Each node (wallet, miner, etc) stores them in a local collection called the &quot;mempool&quot;. The name implies these are typically kept in memory (RAM) but the developers of each wallet software can decide whether to also store the data onto non-volatile storage such as disk.

Can they be modified before a [miner] put them in blockchain

No, because they are each signed
Modifying the transaction would make the transaction signature fail verification.  Only the person who knows the secret number known as a private key can produce a valid signature for a transaction. The inputs to a transaction are locked by a script that can only be satisfied by a number resulting from the use of the private key.
Every other node (wallet, miner etc) would reject a modified transaction and start to suspect the source as being potentially malicious.