how an FTDI chip converts USB signals into JTAG signals
"An FTDI chip" could be any number of things. FTDI makes a lot of different chips, some of which aren't even related to USB.

The "FTDI chip" you're probably most familiar with is the FT232R, which is a pretty basic USB-to-UART part. However, most FTDI-based JTAG devices are based on the FT232H (or the related FT2232H/FT4232H), which supports an interface mode known as MPSSE. The MPSSE mode supports a number of complex synchronous-serial operations, some of which are specifically designed for use with JTAG. For instance, there are a couple of MPSSE operations specifically designed to operate on the JTAG TMS pin.