How does my phone get charged via laptop USB port at lesser current (compared to wall charger)
Batteries can charge at less than the full rate. Just Slower. USB 2.0 supplies up to 500mA standard, after enumeration, not 100mA. Not all usb host devices conform to the 100mA limit without enumeration. Not all usb host devices limit to 500mA either.

Edit due to expanded question: Most new phones (4 years) will adhere somewhat to the USB Battery Charging Spec. For chargers, they will signal what kind of charger they are on the USB Data Line pair (mostly using Voltage Dividers to indicate a specific voltage). Samsung does or used to, have their own resistor pairs, but now mostly uses the standard signaling. Apple did as well, in fact, they are the first to be recognized as doing this. The standard pretty much adopted Apple's idea. Internally, the phones use some type of IC or Microcontroller to limit current.

The three common ratings are Computer (500mA), Wall (700mA to 1A), and High Current (2 A, mostly for iPads and tablets)

For computers, most new phones will enumerate, and draw 500mA. Almost no new phone has charge only usb ports, so that is not the issue. Some phones will treat dummy chargers as computer (500mA) chargers. Some phones will completely refuse to charge from them (iPhones, some Sansung)

So for the most part, no new phone was ever intended to have a non-standard current limiting. They expect to enumerate to a computer or see signaling on the usb pair. And with usb chargers being so common they can be had for single digit dollars in retail stores, there really isn't any excuse for using a 100mA limited supply. But batteries can charge from 100mA, some phones just won't try. Look at the iPad. It is intended to charge from a 2.1A high current charger or computer port with higher current (Which many new macs have). If you connect it to a low current 500mA port, it will SAY "Not Charging" but if you turn the screen off, it will very slowly charge.