I am not entirely sure what I should make out of this response. You are not attacking the position that Ukraine in NATO is a potential security threat to Russia, you are only saying that they are not acting accordingly. Does that imply that you are agreeing that having a competing military alliance at your border is a potential security threat?

Besides that I can think of several reasons why Russia only has fields. Firstly, Ukraine is not yet in NATO, so any defensive measures would be premature. And they invaded Ukraine to maintain that state which seems a way stronger reaction then constructing defensive lines. Also building defensive lines implies worries about a ground invasion and it is not obvious to me that this would be Russia's primary concern.

They might, for example, be mainly worried about the nuclear balance. If the USA could place ABM defense systems closer to Russia, they might be able to more efficiently intercept Russian ICBMs in the boost phase when the missiles are still relatively slow and vulnerable. That would diminish the Russian retaliatory strike threat in case of a US nuclear attack. Also having weapons closer to the target reduces the reaction time the enemy has. For that kind of risk ground defense lines it not the reaction one would expect.

It might also be for simple reasons like a lack of resources. Or Russia might refrain from doing what it does not want NATO to do.

I do not accept your hypothetical scenarios, but I am willing to investigate them.

People often point to Napoleon or Hitler and claim that Russia fears an invasion over the flatlands of Eastern Europe. Yet, if we investigate this, we find that Russia's European neighbors are building defensive lines on their side, while Russia is not. This suggests that Russia is not afraid of a ground invasion, but its neighbors are.

Likewise, if you look at a globe, Ukraine is nowhere near the launch sites or flight paths of Russian nuclear missiles headed for the US. The Earth is not flat. Nuclear missiles would be launched from sites near in central and northern Russia, flying north over the pole toward the US - not west over Ukraine. Ukraine is geographically irrelevant to these flight paths because it is too far south.

So again, how does NATO threaten Russia?

There is one far easier explanation: NATO is not a threat to Russian security but a threat to Russian ambitions. NATO stands in the way of Russia invading other countries. If every country in Europe were isolated and didn't cooperate with others, a Russian invasion would face only their regular forces. For example, Estonia has just a few tens of thousands of soldiers, less than 100 artillery guns, a few HIMARSes, and some anti-ship missiles. No tanks, no air force, no navy. That's it - something Russia could clearly defeat. But with NATO, Russia could potentially face everything up to American carrier groups and nuclear missiles when invading Estonia. Managable risk grows into unlimited risk.

This is self-evident from the invasion of Ukraine as well. Without foreign military aid, Ukraine could have been defeated long ago, but with the aid, Russia is stuck fighting for just the first fifth of Ukraine. Foreign military aid made the difference between victory and defeat for Russia in Ukraine, so naturally, they seek to isolate countries they intend to invade from military partnerships.