I used 50 year old welding leads that H&S retired because of burn marks in the cables casing. The cable was very flexible with a very high strand count of pure copper. The copper was larger than my thumb. I used gator clamps and you could tighten them with a wrench after you clamp on. I’ve seen those cables move several inches when people were trying to jump a heavy load and not charge the batteries a bit before trying.
I learned about soldering jumper cables too. DON’T. I saw a guys service truck burn to the ground. The fire was where his priming fuel and his "jumper cable" connection were. LOL The leads got so hot the solder melted and pored right on the plastic primer container.. Guess what else was in that compartment? The ONLY fire extinguisher. He sure had a clean service truck too. :-) I learned a lot about "Jumper Cables" that day.. It burn both pieces of equipment UP and caused a pretty big grass fire.. He made the news.. million dollar boo boo..
I gave him an A+ at the end of the day, he saved his lunch box..
Regards