What are different battery capacity algorithms used?
There is as far as I know:


Coulomb Counting, where you integrate the current, flowing into or out of the battery.
Voltage Measurement, where you measure the voltage of the battery
IT - Impedance Tracking, 
CEDV - Compensated End of Discharge


Point 3 and 4 are found with TI Chips bq-something.

I know a lot of older E-Bikes use coulomb counting and OCV measurement in parallel, where CC is the primary gauging and OCV a backup verification process.

CC consists of the actual coulomb counting algorithm and a teaching algorithm, which calculates the Full Charge Capacity based upon End of Charge Voltage and End of Discharge Voltage.

Edit:
Cheap chinese manufactured batteries only rely on Voltage measurement, although this method has a large margin of error, depending on the load.