AI is a vast field. LLMs and neural networks are a small part of it.
LLMs are very expensive to run and a lot more complex than the markov chains often used for predictive text.
Predictive text just chooses a likely word based on what’s typed. This may be as simple as looking for words that start with what you’ve typed.
LLMs vectorise words and understand the complex relationship between vectors using many data points. So it would spot the word “two” and realise that plurals are used with it.