I don’t understand the question Vicky. However discharge rates will depend upon the physical properties of the battery, usually the bigger the area, the bigger the current capacity - this is more obvious in lead-acid car batteries. LiPo are made of flexible materials and folded/rolled, so the mechanics are difficult to see. The ability to discharge without the voltage staging is stated as the discharge rate (in C this is multiples of the capacity). You can’t (normally) increase the characteristics of a cell, although you can degrade them by misuse.
A cell is a single unit, a battery is a collection of cells. These can be packed in series to increase voltage, or in parallel to increase current (strings in series can be added in parallel to increase current and voltage). The mechanics/chemistry of the cell dictate the maximum voltage and current available.
Re Arduino, try google. Current is usually measured by taking the voltage across a low value resistor in series with the load, but can also be measured by inductance.
Re-soldering connectors is fine, but you may invalidate any warranty. One can purchase/make converters. Usually you chose a connector based on current carrying needed for your set-up and that becomes your standard.