Fundamentals of electron flow
When we connect the two batteries there's usually a current.  A brief one, then all the potentials on the battery terminals shift, and the current halts. How long does it last?

We can roughly figure it out, since it's just the "RC" equation of capacitor discharge.  The shell of a battery, plus the ground, together they act as a capacitor, and the value is perhaps around 1pF, maybe less. Or, if you're holding the battery in your hand, maybe it's higher: 10pF?  Next, the connection between batteries is a tiny fraction of an ohm.  So, when you connect the batteries, the major part of the current lasts for R*C seconds.   That's 0.01ohm * 1*10^-12farad, or a hundredth of a picosecond.  Not very long.  (The short pulse probably makes some radio waves, GHz radiation!)

Once the discharge has occurred, the potential between the connected terminals of the batteries is zero.  This means that the other two terminals now have ~3V between (for 1.5v zinc cells.)  Huh, so, what then did they have before touching?!  Good question.

A battery sitting on a table might be charged to a few hundred volts WRT earth.  In winter this could easily be higher: a couple thousand volts.  It's caused by handling the battery with dry fingers, or by placing the battery on an insulating surface which earlier had been brushed by clothing or skin.  The entire battery acts like a single charged object, like one capacitor-plate, with the second plate being the earth, or nearby conductors, humans, etc.  In other words (guess!) the potential between two separate batteries could easily be several hundred or thousand volts, with the polarity being random and unknown.  If a D-cell happens to have about +50V on it WRT earth, then one terminal might be +49.25V, the other +50.75, for 1.5V between terminals, but 50V on the battery as a whole.  Touch this to another D-cell which has, say, -632V on it, then figure out how the potentials change!

Aaaand, the same applies to dead batteries, where the voltage between each battery's terminals is zero, yet the battery may have hundreds of "floating volts" on it WRT earth.


  When lightning strikes, where is the loop?


That's just capacitor-discharge.   In thunderstorms, the regions of charged droplets become the "capacitor plates," and become conductors when the enormous streamers/fans of plasma connect them all together.  Imagine two highly-charged soup-cans sitting on a table, with a few tens of kilovolts between them.  Push them together, a spark jumps between.  Where was the loop?  Part of the loop is in space, in the collapsing e-fields found between opposite-charged cans.  Same with thunderstorms: as the e-fields collapse between the opposite-charged regions, the fast-changing fields behave like an electric current.  Search on Maxwell's equations, and look for Maxwell's great intellectual breakthrough: "displacement currents" in empty space.   When we charge/discharge capacitors, the path for current goes through the capacitor.  During lightning, the "loop" is completed through the capacitor dielectric, inside those collapsing e-fields located between the opposite-charge regions in the cloud.


  touch 220V line


Usually we're not shocked by touching live AC house-wiring.  (BUT DON'T TRY THE EXPERIMENT!)  Dry floors are going to have many megohms or even gigohms resistance to any earth connection.  If you touch a single 220V line with one finger, you'll just get some nanoamperes of capacitor-based current, where your body is one capacitor plate, and the earth is the second plate.  The danger comes from low-resistance connections between your body and the ground.  These are common, and famous in electrocution accidents: going barefoot on wet concrete basement floors, wearing wet shoes outdoors on wet dirt, sitting in a full bathtub, grasping a metal electrical conduit or an appliance, or leaning on grounded objects such as metal sinks.  See this semi-complete list!


  Current is flow of electrons


Nope.  Currents in metal wires are flows of electrons.  That's a technician viewpoint, but the view of scientist/physicists is different.  In general, electric currents are flows of electric charges including ions and protons. For example, currents in the ground are flows of protons (called +H hydrogen ions,) plus sodium ions, chloride ions, -OH ions, and misc. other metal ions (potassium etc.)  No electrons.  When you get a shock, no electrons flow through your body, and the current is a flow of the same ions found in dirt.  Human brains and nerves aren't based on electron currents, instead they're flows of +Na, +K, and -Cl ions.

And, the currents in acids are flows of acid's primary mobile charge, the +H ion.  Proton flows!  Inside the car batteries are hundreds of amperes of proton-flow.