Looking for answers to some basic questions related to transaction and upgradation.

Hi everyone, please explain me the needed in the simplest manner.

1) In a transaction's lifecycle which I assume is as follows:

Sender signs > the message goes to first node's mempool > the first node includes it into a block > sends the block to other nodes in the network > once over 6 nodes have added that block to their blockchain copy we can say it is impossible to revert that or to fail ... the above process repeats every 12 or so seconds from the part where block comes into play, the mempool is an area for txns to wait until they get included in a block.

I want to know,

a - Between which two stages is the 10 checks for mempool done and also who does it?

b - How is the token smart contract involved and used in this transaction? Please share easy to understand but a slightly more detailed explanation than 'contract is called and node runs blockchain on which it exists'.

c - What decides which node will receive a particular transaction?

d - Also if the txns are ordered in the mempool based on total gas and tip associated or on the timestamp mentioned for the signature then how does the order of transactions become non predictable?

2) How are changes made to the immutable code running as a smart contract on the blockchain? I understand how eips and ercs are proposed and approved at various level and finally implemented but is it all the time a new separate smart contract with whatever minute change may make it new? also how does the whole community switch to the new contract if that is what happens?