How to understand flavors and when to use combination of them?

Yes, all dishes should have flavour combinations, unless the dish consists of one single ingredient with no seasoning added, no oil added, it can't help but have them... even then a tomato for example has different flavour in the skin than it has in the pulp, than it has in the seeds, the inner leaves of a brussels sprouts will have less bitterness and more sweetness than its darker outer leaves... flavour combinations are almost impossible to avoid.

So to balance flavours you do have to understand them. Not everyone might agree with the combinations you choose to create and not everyone enjoys the same taste combinations, but even so, to reliably create the blend you do like, you need to understand how it is composed.

There are any number of resources online which will break flavours down to a few key groups, though not everyone describes it the same way. Essentially you need to understand what is meant by basic terms such as sweet, bitter, sour, umami and salt. To those five you can also add 'spiciness/heat' which is often considered to be more sensation than flavour.

You should read up on what it already accepted knowledge about the effects these flavours have on each other, how salt changes perception of bitterness, how sweetness can counteract excess salt but leave the umami clear. Read up what chefs have to say about these interactions and test them out, see if you detect the same effects they do.

Spend time tasting your ingredients and training your palate (I was so busy thinking of flavours as a palette from which one can choose the equivalent of colours to paint a dish as you might paint a picture that I originally spelled 'palate' as 'palette') so that you can analyse a dish and detect what makes the difference between a combination you like and one you don't. Understand your ingredients, both fresh and storecupboard ones well enough to know quickly what will make that difference.

No, not all dishes need to have flavours combinations. I would also say that it's not about "sweet and hot".
IMHO it's more about particular flavour. Like when people drink wine and say "an earthy flavour with a note of pineapples and just a hint of pining for the fiords".
Knowing the flavours means that you know what impact have different ingredients on the overall taste.
For example you might not add nuts for nutty flavour but replace it with chickpeas. Or that, in certain parts of the world, any combination of two out of "five Chinese spices" will give your dish "Asian taste".

Like GdD wrote in their comment. You need to taste a lot of food, spices, herbs, make a lot of combinations and from that be able to know what you can mix to achieve certain goal.

To try take one small aspect of a really really broad topic...

Try making the simplest salad in the world.
4 tomatoes, 1 onion. Chop into chunks & put in a bowl.
A little salt & pepper & it's done.

..but wait - over there we have a choice of three fresh herbs we could add.

Cilantro [coriander], flat-leaf parsley or basil.

Add cilantro it's instantly 'Mexican'
Add flat-leaf parsley & it's 'Turkish' or at least 'Mediterranean'.
Add basil & it's 'Italian'.

So with just three ingredients we have three recognisably different cuisine styles.

You could easily dress those to accentuate each cuisine.
Mexican would take lime juice nicely.
Mediterranean, olive oil, vinegar or indeed lime juice again.
Italian, oil & vinegar.

This is, of course, vastly over-simplified - but I bet even done as simply as that, each would be tasty ;)

In terms of just flavors, get to know each flavor on its own and then you'll be better equipped to combine them in ways that show you "understand flavor". Like colors to a painter, or sounds to a musician. When you taste things by themselves, you begin to develop an intuition as to what to add to what to have the outcome you're looking for.

You want to be able to ask/answer yourself things like: what does garlic bring that onion doesn't, what does ginger bring that cayenne doesn't or why would you choose to sweeten with agave syrup instead of muscovado sugar (or vice-versa)? Knowing what each flavor brings to the party (even if it's subtle) is literally what it means to "understand flavor".

When you're doing it right, it blows people's minds :)