When LCDs update, do they go line by line like CRT? I'm just trying to understand if tearing can occur with LCDs. Is double buffering necessary for them? In CRT there is vertical blank interval, do LCDs have something similar?

No, there's no vertical redraw with an LCD. Instead LCDs have response times as part of their specs. The faster the response time (listed in milliseconds) the faster they redraw and the less streaking you see. However companies measure these response times differently so you can expect like a 2-3ms variation perhaps.

However you still need a back buffer

There is absolutely vertical redraw on LCDs.

They get a sequential, line-based signal just like any other output device. "Tearing" actually happens on the graphics card (when the data changes under the "read pointer" on the card), so it will be visible on all output devices.

A slow LCD might smear the output in time enough that the tearing doesn't look as bad, but it's still there.