(for TWRP): Power off the device, then press and hold Power + Volume Up + Home . Common Sources for ROMs
boot.img contains kernel and init scripts. Use AIK:
Download the official firmware (e.g., T113XXU0APB1 ). Use a tool like ImgRePackerRK to extract the contents. You are looking for the system.img or factoryfs.img file.
./unpackimg.sh boot.img
The most stable and popular "repacks" or builds for this device include: LineageOS 14.1 (Android 7.1.2 Nougat)