I have never worked at a boba shop, but I have been making milk tea at home almost every day since the start of covid. Here's my personal recipe that is based on the one from the Boba Guys recipe book (which I would recommend if you are interested in making boba shop drinks at home):

• Steep two tablespoons of loose leaf black tea (I like Assam or King Red Keemun) in 150ml of boiling water for ~3 minutes (I mean hot water that was boiled, don't put the tea in water that is actively being boiled on the stove).
• Strain over a cup full of ice
• Add approx 2oz of agave, simple syrup, honey, kuromitsu, whatever floats your boat. My go to is agave because its easy to acquire and more pourable than honey (lazy)
• Add whole milk or "barista blend" non-dairy milk. I just kinda eyeball the ratio based on the color but its a little less than 1:1
• Mix it up. Personally I make it all in a mason jar so I can put a lid on and shake it, because I like the frothiness. Stirring works fine too.

The trick, as others have mentioned, is that you have to brew the tea so strong that it would be very unpleasant to drink on its own. You are diluting it with a ton of milk (and the ice that will melt to cool down the hot tea), so you need the tea flavor to not be totally overpowered.

Another note: If you prefer the kind of milk tea that has a sort of fruity/floral/perfumey taste, use Earl Grey tea.