Non-mining nodes relay blocks according to their consensus rules,
they are nothing more than a personal copy of the blockchain / transaction receipts.
Non-mining nodes can only relay block data, Validation & Creation of blocks only occurs on Mining nodes.
Want proof , turn off your non-mining nodes, any effect of the network will not even be noticed.
Network will continue on , normally because the mining nodes do all of the work.
mining nodes:
validate
collate(block create)
relay
non-mining nodes:
validate
relay
take coinbase.com. get them to turn off their non-mining nodes and you will soon realise that MINING NODES cant spend their blockrewards on coinbase.com because coinbase.com cant see or trust that a miner has value to spend.
non mining nodes do validate.
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how ever there are some software that just relay. such as matt corrallos 'fibre' and also some lite nodes designed specifically to just relay out data unchecked independantly.
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also.
if one mining node made block X with an error
if one mining node made block Y with without an error
with coinbase.com's NON-mining node. would validate both attempts(if both received near same time and Y has not yet been built on to be defined as a clear winner) and when when validated deem block Y the winner. and as such Block Y can happily be placed as the next block coinbase keeps. thus allowing the miner of Y to spend the block reward. where as mining node X will get forgot/rejected
....
also
because coinbase.com has a chain that includes block Y.. and other peers see that coinbase and N other peers also have Y then the consensus is that the chain containing block Y is more acceptable to everyone compared to that the one node trying endlessly to push block X. eventually the one node endlessly pushing X would get banned from being a connected node if it made too many push requests
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also
whn the MINING nodes want to make the next block. they see many peers have chosen Y. and thus its best they build a block on Y to avoid orphan event
.. also
non-mining nodes do check with their peers what best chaintip, height and blockhash to follow and wouldnt really bother even trying to look at a chain containing X if Y has more acceptance amongst peers and also Y is ahead with other blocks now above it and the network building on further.