They're absolutely relevant at your level. Most moves don't qualify as a tesuji, but a lot of basic combinations that you may already play have tesuji lurking in the background. It's a common situation for a joseki move to be right because playing a different way is refuted by a particular tesuji.

You don't, or at least I don't, typically think of them as tesuji in games. You just read and try to find the move that works.

Tesuji are not something you normal go out of your way to play. Tesuji problems train you to look at the vital points of shapes. Sometimes one point is key, sometimes it is another. What you are doing by practicing tesuji problems is learning to recognize key points when they occur in your games.

So don't go looking for tesuji. Go looking for key points. Those will often correspond to a tesuji problem you have previously countered. But key points may be strategic as well a tactical and it is only the latter which are usually called tesuji.

I guess I'll have to make your word for it on this one. All the strong players keep telling me that I would be able to recognize key points and shapes in the future, but it boggles my mind how that is possible given all the situations and scenarios. Even similar situations sometimes seem to have a different key point or solution because of one intricate detail.

It is not as bad as it seems. Think about the analogy with learning a language. young kids learn their letters but don't know their words. After a little while they learn their words but still have trouble reading complete sentences fluently. At some point in their development they learn how to pronounce though, bough, thorough, thought, etc.

Development of your instincts for tesuji works along the same lines. All it takes is practice.

Are tesuji problems useful to a kyu player . . . ?

Looking at tesuji problems these days have me looking for a way to use them in my games. In reality, how much should I be thinking about them in my games? All the time? Once every 20 moves? Don't even worry about it at my level?

is there a book that would teach me the "alphabet"?

Is there a tesuji in every situation?

Is there a book that would teach me the "alphabet"? Although, it seems more like Chinese characters to me.

Looking at tesuji problems these days have me looking for a way to use them in my games. In reality, how much should I be thinking about them in my games? All the time? Once every 20 moves? Don't even worry about it at my level?

Are tesuji problems useful to a kyu player or is this one of those things reserved for higher level players? It's confusing since it is talked about so often in commentaries and lessons.