What meanings do the "NodeId", "Version" and "Services" convey on Peers tab of Bitcoin-qt Debug window? As shown in the picture,

Is "NodeId" the ordered number (among all the existing bitcoin-core nodes) of the node that I am connected to? 

How does "Version" differ from the "User Agent" version (on the picture, Version is displayed as 70015 and User Agent is displayed as "/Satoshi:0.18.0/)"? 

And lastly, what does the "Service" mean (in this case displayed as "NETWORK &amp; BLOOM &amp; WITN...")?
Is "NodeId" the ordered number (among all the existing bitcoin-core nodes) of the node that I am connected to? 


It's just the number of the connection you made. It's not a global identifier of the node you're connected to. Every new connection you make (incoming or outgoing) is assigned a sequentially increasing number.


  How does "Version" differ from the "User Agent" version (on the picture, Version is displayed as 70015 and User Agent is displayed as "/Satoshi:0.18.0/)"? 


Version is the protocol version. User agent identifies the software implementing it.


  And lastly, what does the "Service" mean (in this case displayed as "NETWORK &amp; BLOOM &amp; WITN...")?


The Bitcoin P2P protocol has a number of optional features that node can offer to their peers:


NETWORK means availability of all blocks since genesis
NETWORK_LIMITED means availability of the last 288 blocks
BLOOM means BIP37 bloom filters
WITNESS means support for Segwit witnesses in blocks and transactions
COMPACT_FILTERS means support for the BIP158 basic filter type