The original design was by Wolfgang "Wolf" Büscher, DL4YHF, back in 2006, and is brilliant. Whoever ripped off his design to start selling them from China added a pretty terrible crystal tester - a single transistor Colpitts oscillator that runs from the unregulated side of the power supply, and has poorly chosen component values.
Various YouTubers have hacked the pretty useless crystal tester section, into a useful signal conditioner and preamplifer, using the kit-supplied transistor plus a few standard value resistors and an unpolarized DC blocking capacitor (0.47uF suggested, but I didn't have one and used a 0.1 uF, as I don't really need to work down to very low frequencies).
YouTube channel, TheHWcave, made an excellent video showing his modifications to the board, and has also improved the firmware, tweaking it to cover the range of frequencies from D.C. up to 120 MHz. But if you don't have the equipment to re-flash the PIC, the program supplied in it, in the kit, is the original from Wolf, and works fine up to about 70 MHz.
The design uses a PIC16F628A processor, and impressively manages to drive five, seven-segment digits, plus decimal points, from an eighteen-pin chip. When you consider that the chip needs two pins for power, two pins for its own crystal, one pin for the signal being measured, and one for the push-button, that only leaves twelve pins to drive the multiplexed display! It doesn't use any external logic chips, just a few diodes and a transistor to craftily control five digits using four pins.