Your decoder would then look at the vertical blanking lines with the data on, and wait for the page to match the one you selected. This could be fast (typically pages like 100 would be broadcast every second or so), or slow (page 693 might be sent once every 15 seconds)
The decoder would then show it. The decoder would only need a 1 page memory.
"Read only" teletext didn't include a decoder at all, it was the output of a decoder at the TV station which was sent via normal UHF means, typically with background music.
As memory got cheaper, "Fast Text" was introduced, each page would have shortcuts to 4 more pages (red, green, yellow, blue). These would then be stored in the background when they were broadcast, so the decoder needed 5 pages of memory, but the experience of the user was much better as there was less waiting for the next page they'd usually want. This also allowed "hidden" pages like "35F" to be used.
Towards the end of analog programming, teletext brought in on a PC capture card was stored and decoded by your media player. Computers obviously had enough memory to store every page so response was instant.