Whether we can use chemical and organic fertiliser both for bonsai if so how it should be used?

As with many questions on fertilizers or bonsai in general "it depends".
In general practitioners (myself included) tend to rely more on organic ones, particularly in form of cakes.
Multiple reasons for that, starting from the fact that they feed the soil slowly over time.
But it's not like you can't use synthetic ones and obtain the same results...

As to "how it should be used", again the answer is "depends".

In general according to instructions coming with the product, but different stages of bonsai development require different fertilization strategies. So do different specimens, so do different climates, ...

Way too many variable to respond in a blog post.
If you have questions, I'd suggest state your situation (climate/location, type of plants and development stage, ...) and ask about what you want/need to know specifically.

Different people do different things, for various reasons. Solid fertilizer works real well for some, and so does liquid.

I think solid is better because it releases nutrients slowly and it seems to me that pouring water on the soil the day after fertilizing just washes it out of the soil. I have no evidence for this thought. It stands to reason, and that is a formula for some incredible blunders throughout history.

In the US the term "organic" used in advertising or product names has little meaning and usually means "more expensive ". Other jurisdictions may vary.

I use both.

Where organic to me means, fertilizer that is composed of organic compounds (Carbon-based, normally plant material, and or animal exrements), vss the chemical fertilizer, which normally consists of the salts of sodium, iron, magnesium etc with nitrates, posphates etc.

The organics will help sustain bacteria and fungi in the substrate which in the long run seems to keep plants healthier. For this reason I will always have an organic component in my substrate & my fertilizer regime.

A friend that own a grow shop once told me that chemical fertilizers kill the organic bacteria in the soil, he told me a long time ago and I dont remember in what context but I thought I would share and maybe someone could elaborate this more or deny it completely.

Enaisio wrote: A friend that own a grow shop once told me that chemical fertilizers kill the organic bacteria in the soil, he told me a long time ago and I dont remember in what context but I thought I would share and maybe someone could elaborate this more or deny it completely.

Again, it depends.
Fertilizers contain nitrogen, phosphorus, and potassium, but they are in some compounds... Nitrogen in its elemental form is gas (most of the air is nitrogen), but plants can’t catch it from the air. You also find lots of nitrogen in excrement (liquid and solid), which is a form more accessible to plants. In excrement, nitrogen is bound in some nitrites, but there is also all kinds of other stuff in there. Chemical fertilizer could be for example some typical nitrite you find in sewage, but in pure form. In if you apply too much, it probably will kill bacteria, fungi, and your plants. From the natural sources it takes some time for them to decompose into the chemicals the plants feed on, and the necessary nutrients are released little by little. It’s similar to animals using sugar as fuel: from fruit you get some, but it’s difficult to eat too much, but eating too much pure sugar is quite easy and can make us sick. Then again, in moderation sugars work just fine as fuel for animals.

So, I if you know the correct dosage, chemical fertilizers will not kill anything.

Most of the nitrogen in excrements is in ammonia form.

The most common nitrigen source in chemical fertilizer is amonium nitrate.

Nitrite is a poisonous form of nitrogen and just a step in the nitrogen cycle, I have never heard that there are nitrite in and excrements or commercial fertilizer. Not sure if most plants can use it.

Enaisio wrote: A friend that own a grow shop once told me that chemical fertilizers kill the organic bacteria in the soil, he told me a long time ago and I dont remember in what context but I thought I would share and maybe someone could elaborate this more or deny it completely.

As far as I can tell, the problem is more that if you only fertilize with chemical fertilizer, there is nothing to breakdown for the bacteria. So they starve to death. The chemical fertilizer contains the elements in its most basic form available for direct uptake by the plants. It exists in this form throughout nature too. It is not poison.

A lot of people use "organic" fertilizers exclusively. Most of them have a lot of healthy plants.

A lot of people use "chemical"fertilizers exclusively. Most of them have a lot of healthy plants.

It really doesn't matter.