FLIRC is a product, purchasable on Amazon. Basically it's an IR-to-USB Keyboard dongle. It can be programmed to send any keystroke when it receives any IR command. It's benefit is it has extreme customizability and excellent performance, and it's compatible with anything that can accept standard USB keyboard input.

Its name stands for "F*ck LIRC", where LIRC stands for Linux Infrared Remote Control, the system process used for receiving and processing IR commands from add-on or built-in IR sensors on numerous Linux-based computers, TV boxes, media players, etc. LIRC is notorious for being difficult to configure/customize and for having laggy / erratic performance.

OTG has already been explained, but basically it's a cable that allows something that's normally a USB device (like a cell phone) to act as a host (like a laptop / computer) and accept accessory devices (like keyboards, cameras, game controllers, etc.)

So, basically, it would be a cable that adapts a Female USB-C or Female Micro USB socket, into a Female USB-A socket.

In terms of the cable needed for the FireTV stick, as the FireTV only has a single Micro USB port and it's used to power the device, you need a special Y-cable that splits off a female Micro USB jack so that power can continue to be supplied while your accessory device is plugged in.