I have a 2014 Civic with push to start ignition. Car was left in R when I turned it off in the morning. Came back at night, shifted into P, and the ignition button wouldn't start the car. No starter motor sounds or any apparent response. Some interior lights still worked (e.g. lights in the overhead console), and power locks worked.

I wasn't sure if it was battery related since I always thought dead battery meant no lights at all. Still, I decided to try jump starting the car. Got a helpful passerby to pull his car up to mine and I busted out my jumper cables. Followed the instructions in the manual for connecting:
1) red to dead battery (+)
2) red to donor battery (+)
3) black to donor battery (-)
4) black to dead engine block

Saw a spark when I connected the clamp in #4, thought all was good.

We got the donor car started. Let it run for a few mins. Hit the ignition button, but still no signs of life. Waited a few more minutes, tried again, and still nothing. I was starting to think it was something else, perhaps a blown fuse etc. We stopped the donor car.

Another helpful citizen came by and suggested we connect the black clamp from the dead car engine block to the dead battery (-) terminal instead. I recall reading this was ill-advised and dangerous so I caught his hand and asked him if he was sure. He seemed confident, so I let him do proceed after I backed away a good distance. My life flashed before my eyes as he made the connection. Nothing exciting happened. We got the donor car to start up again and jumped into the Civic. As soon as we hit the ignition button, the car came alive.

I drove around for 30 mins before pulling into my garage. I took a digital multimeter to the car and checked the voltage across the (+) terminal and the engine block I clamped to and it read 12V.

What happened? Doesn't that mean that connecting to the engine block should have worked?

Lingering questions:
1) Did leaving my car in R drain the battery?
2) Why did connecting black to dead battery (-) work while connecting black to engine block not work? I thought it's the same ground.

Any insight is appreciated!

I've always connected both leads to the battery and never had a problem

If the battery looked questionable, or if it was really dead then I'd probably ground on the block.

All the resources I've read so far has cautioned against connecting directly to the dead (-).

e.g.
jumper cable instructions,
Civic car manual,
https://www.dummies.com/home-garden/...p-start-a-car/
https://www.familyhandyman.com/autom...ur-car-safely/

It appears to be due to danger of the sparks causing the battery to explode (from last resource above).

you do as you wish

The block is not always a good ground source especially on an older car with corrosion and not ideal connections. I have always connected straight to the battery (yes my jumper cables say to use the block). Battery explosion highly unlikely!! Batteries don’t just blow up (Lithium is more likely on impact or with improper charging) not regular lead acid or AGM.

They are going to go with the more cautious thing to do.

Simple electronics.

The path of least resistance, loses the least amount of energy while charging. Therefore direct connection to the battery is always the best option for jumping, energy wise.

If you have a servicable battery leaking hydrogen then it cold flash ignite the gas escaping. But the service access points will blow off, the battery wont explode, explode.

If the hood is open in a ventalated area the hydrogen shouldn't pool enough to flash.

The interior car lights worked because they draw less currant than what is needed to start the vehicle (probably LED lights)

Appreciate the insight. I'm surprised that all the literature I've found says never to do direct to dead (-) connection but the responses from users say otherwise.