There's no interdiction capability that can be relied on.

No matter where you find yourself on that hot day, many cold ones will follow. The food supply will dwindle to a small fraction of what the survivors will require. Being underground will not fix that.

There's no hiding from humanity's nuclear suicide attempt. Do what you think best to prevent it.

No one is launching nukes. Stop letting the news terrorize you.

A (modern) ballistic missile is basically a rocket carrying a few nuclear warhead to space and then falling from space back to earth, and very high speed while splitting the warhead on the way down to hit several targets (while some of these warhead may-be dummy). And this let alone all the fancy hypersonic ballistic missile which are steered by "bouncing atop the upper atmosphere* making them less predictible while travelling even faster.
So basically, we won't know the exact target before the last moments of the flight, letting may-be a few minutes to the inhabitant to hide.

Regarding "anti missile defence" I assume that the people who knows the details won't post about-it on lemmy. What you need to remember is that even if you catch 80% of the missile falling (let's be optimistic), it will still do a lot of damage.
To take French arsenal, according to Wikipedia, a Triomphant class submarine can carry 16 M51 ballistic missile, each of them carrying 6 warhead. A submarine emptying it's whole load on a hostile country would shoot like 100 warheads. Assuming that air-defence can intercept 80% of them, it's 20 nuke exploding. I can't think about any country which would resist to getting 20 cities wiped out of the map.

So yes, if nukes start to fly, we're fucked up.

I think of it this way.

The people who would actually launch the missiles would be the same people who told Putin that the Ukraine invasion would be a walk over.

I think the chances are much higher that the Army would kill Putin long before they would risk losing their nice apartments in Paris and London.

That kremlin retard says a lot of things. Don't validate him.

Step 1 - Putin doesn't like a thing.
Step 2 - Putin says that if you do the thing you'll get nuked.
Step 3 - Putin's little cyber puppets flood social media with posts "concerned" about the risks of him launching the mighty nukes. Oh noes!
Step 4 - Putin hopes you believe him.
Step 5 - Nobody believes him.

...some times passes...

aaand we're back to Step 1...

How deep underground is safe to protect against a modern nuclear blast?

It depends if you live in St. Petersburg or in some bunker in the Urals. If you're in St. Petersburg, then it doesn't matter; but you should be pretty safe in the Urals.

I don't really know anything on the subject, but imo time is your enemy there, not distance. You won't get far enough away, so as you mention you're looking at going underground (assuming you already have that set up). But how long can you last down there? Probably not long enough.

Unlikely to happen but global warming and general environmental pollution is going on now. reduce, reuse, recycle

First, Russia is unlikely to nuke anyone, assuming of course that their nukes actually work. The consequences for a nation that did decide to let off the big firecrackers…. Well, no one has a large enough army or enough nuclear weapons to fight the rest of the world. Or rather, what would remain of it, and the Geneva Conventions would become more the Geneva Suggestions.

As for warning time, that depends on the deployment method. Tactical weapons, you wouldn’t have much, if any, warning. Strategic weapons, anywhere from 5 minutes to a few hours, depending. Tactical weapons tend to have ranges somewhere in the 100 km range, and have, generally a much smaller blast radius.

Interceptors have been developed, (s400, Patriot and others) but how well they would work against a nuclear warhead is untested as far as I know. I think it would depend on how the warhead was fused and triggered.

As for your last question, no clue. My understanding that current warheads range in size from something that can take out a city block to weapons that can take out New York City and everything in between. The only thing the average person can really do is treat it like you would a tornado. Head to your nearest shelter and stay away from windows.

Dr. Tyson says that modern nukes don’t usually have much radiation fallout anymore but still, be upwind if you can and take a shower after the attack but don’t use conditioner.

If the thought of possible nuclear war scares you, you might want to consider learning more about it. There are two channels on YouTube I would recommend as a starting off point, Perun and Kyle Hill. Perun does military economic analysis and Kyle Hill is a science educator with an emphasis of nuclear technology. Both have done presentations on nuclear weapons and their use.

Before hypersonic weapons, it was accepted the US gets at least 5-30 mins with the first hits from submarine launched missiles, the EU less than 1-4 mins.

Now? Does it matter?