Dragon fruit have to fully ripen on the cactus. The tendril or dragon scale like bits will start to brown and that's my favourite time to pick at least. The yellow kind I just guess because they don't have those. They don't ripen after being picked and will taste like dirt if picked early. If you have the climate or maybe greenhouse you can grow them by planting them in a pot held up at 8 ft or higher with the cactus segments dangling down. The fruit grow from the hanging parts. I still have a bunch but they need more nutrients to fruit any more and I've been too lazy to go outside to buy the fertilizer. Also post lockdown anxiety.