connecting a circuit to device through 3.5mm jack
No, different devices with have a slightly different peak voltage or frequency response. But they will be similar enough. An op amp is sufficient to fix any low signal. Or the gain setting on your pc audio card. 

The standard exist, and it's headphone or line out. Pretty much standardized based on decades of consumer electronics production 

Maximum current is not an issue here. These are logic level signals and not intended to be power sources. The current drawn is milli or micro amps, and power is milli Watts. This is enough to drive headphones that are a killaohm,  or megaohm ICs. Anything bigger needs an active/powered amplifier. 

Generally speaking, yes.  TRS connections have been standardized. Left Right Ground.  TRRS connections build an extra conductor, and have two main standards, the Apple and the Everyone Else standard. The two swap the mic and Ground connections.