MBP won't turn on - need help for hardware troubleshooting

Hi, I have an issue with my Mac Book Pro Retina Late 2013 - It wont start up. Here is a description of the PROBLEM:

• When I try to turn it on nothing happens. No chime, no nothing.
• Charging lights indicates the battery is charging (orange)
• When performing System Management Controller Firmware reset (Shift+Control+Option+Power button) the light indicator on the charger turns green, then orange again)
• I have also tried NVRAM reset among other “tricks” that involve pressing keys
• The last bit of information that might be useful is that it worked just fine, I closed it and later that day I plugged a USB cable to charge my watch. I remember if the MacBook was charging at that point. The watch was charged only a few percentages. The next day it didn’t turn on anymore

My proposed NEXT STEPS:

Next I would like to check the hardware components but I need some guidance to make sure I do the right things.

First I would try shorting the power pads (found here) to check if it’s the power button that doesn’t work.

Q: Is there another way to start it up?

Next, I would like to check the battery.

Q: Is there a way to check it without having a replacement already?

Q: Should the MBP start only with the cable plugged in

To check the SDD;
Q: Can I boot my other Macbook Pro (2021) from the hard drive of the old Mac Book?

And lastly;
Q: How can I check if it’s the mainboard?

Also, I’m open to suggestions on how to approach this differently !!!

I’ve got the tools, maybe a bit of talent but very little experience. But I’m eager to give my best and learn :)

Thanks for your help!
Tobi