Hi, I'm new to this forum, I don't need help with a certain puzzle, rather with how to improve my technique. I've always liked sudokus but only got deeper into it recently. It started when I found a puzzle on some website when I was bored and I couldn't solve it(which used to be quite rare, I could solve most of them in newspapers). I found an online solver and when I loaded it the solver started pointing out answers using terminology that was unknown to me, like XY-Chains and Jellyfish and the others. I understood its logic though and found it fascinating so I've decided to read up more about sudoku solving techniques and I've also found a few websites which offer quite difficult puzzles, well, at least for me.

My main problem is that with these very hard/diabolic puzzles, at some point I always get stuck. Sometimes it's because I'm searching for too difficult stuff and I fail to notice a simple XY-Wing or even just simple pointing pairs. However, more often than not, what I'm missing are some kinds of chains. Simple colouring or XY-Chains, etc. I have a really hard time finding these because I don't know where to search for them because sure there are many chains at any point on a grid, but most of them are not really useful.

I mean, an X-wing is really easy to find, I just go through the grid line-by-line and column-by-column, and if I find a number with only 2 instances in a unit, I will look for another wing in other units, it also turns out pretty fast whether it's a useful X-wing for elimination or not.

But with these chains, how do I know that a chain will turn out to be useful? Is it just trial and error, like "Well, here's a chain, let's see if it eliminates something"? Or, is it more like "Damn, it'd be so good if X wouldn't be possible in this cell, maybe there's a chain to kill it"? Either way, there seems to be quite an amount of intuitive factors involved. Do I just need to start solving a mass of puzzles so I get a bit more experience with the more difficult ones, or is there a shortcut, like some theory about in what kind of circumstances should I be looking for a chain?

Here's a quick example:

On Diagram A, you can see the point where I got stuck. I loaded the position into a solver, and it pointed out an XY-chain, demonstrated by Diagram B. The chain starts from r7c7 and ends at r8c3. A 4 from r8c7 and another from r8c8 can be eliminated which will lead to naked pairs and basically almost solves the entire puzzle, only an X-wing and a couple of XY-wings are left before the grid is reduced to a bunch of naked singles eliminating each other. On Diagram C, there's a set of chains, originating from r5c9, but they're completely useless for the moment, unless I'm planning on solving half of the puzzle in my head.

So basically my question is, how should I know that r8c7 is a critical cell and r5c9 is not? As far as my newbie knowledge goes, both seem to be reasonable options at first glance.

Thanks, in advance.