For thorough answers, you should split your questions up (and search, as these questions have answers on this site already, but perhaps you don't really now what to look for). Therefore, I'll try to give short answers here:
- Suppose a node doesn't have internet connection then it would not receive the broadcast of other nodes then does the blockchain propagates without the validation of that node ( that is, is it 51 % of all nodes or 51 % of all active nodes )**
The percentage of nodes enforcing rules and observing the chain doesn't matter, as long as you use your own own fully-validating one (it validates everything other nodes tell it). What matters for 51% attacks is miners (and specifically, their hashrate). If a miner is offline then indeed their hashrate doesn't matter.
- If i have a bitcoin wallet , but that doesn't mean i have entire copy of bitcoin blockchain (~250 GB) right ? , i have read about full node, partial node but still don't have clear understanding.
Some wallets are part of the node implementation (like Bitcoin Core), others aren't (like Electrum). In case you don't have your own node, your wallet will inevitably be connecting some someone else's node, which implies placing some trust in the party that provides that service.
Among nodes there are different levels. There are fully-validating nodes or simplified validation nodes (SPV). The latter again rely to some extent rely on other nodes for validation, and indirectly, keeping miners honest.
Among fully-validating nodes you have have nodes that store the blockchain fully, or those that prune (which means deleting old blockchain data after it has been validated, but still downloading everything).
- what is the difference between validation that miners perform and all other common nodes perform
Ideally, none. Miners are supposed to be fully-validating nodes, as that prevents them from wasting time and money trying to mine a block that the rest of the network will find invalid. In practice, miners do sometimes create blocks without fully validating, in order to quickly construct a new block after another miner found one, without needing to wait to see the full block.
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