Doubts regarding charging my phone

I am sorry to differ from the views expressed in the above response. Using a short or long cable should practically have no bearing on the rate of charging. The quality of the cables ( or the wire used inside it, to be more precise) holds the key to the charging efficiency of the replacement cable. Go for renowned brands rather than looking for a cable at the local stores. The only point you need to note down is that there should be no resistance to the flow of current. The low-quality build can result in the high resistance and that is exactly what causes issues in charging rate.
As for the length of the charging cable, I have used cables that have even 5 meter length and offering great deal of performance as long as faster charging is concerened. Yes, pay attention to the fact that cable extenders may slow down the rate of charging. Never ever use them. Since you are adding a resistance by means of the additional extender joint, the flow of current will reduce.
However, for all the practical uses, a charging cable that has a length of under 3 meters should be the ideal one. Please note that the cables come in feet and an ideal cable should be 10 feet in length.
There is nothing unique in cable with respect to the phone you are using it with. All the phones from a single manufacturer will almost have the same cables and interchanging them will have no effect. The point in consideration is the charger you use. If you are going for a replacement cable, go for the best brands and check the guage of the wire used if you can.

Yes, surely you can use any USB cable for charging your smartphone. The cables available today in the market are generally universal, so you can buy any USB and use it. It is such that just to provide the customers with a complete package, the manufacturers of the phone will provide you with these accessories. The only difference could be with the compatibility of OTG, which you need to check out with your phone and the cable that you purchase, nut apart from that, it is only the quality of the cable that matters.
Now going for a renowned brand ensures that you get an unused new one, but in local stores, there is no such guarantee, the shopkeeper may hand over to you an used one, which he has bought second hand, and in that case, you could be at loss.
Next, comes the length issue, it is all the same. Nowadays, to avoid tangles, the manufacturers are making the wires shorter, as the more it gets tangled, the more connectivity and electricity flow gets disrupted. SO, while choosing, it is advisable, not to go for too long ones.
Coming to your next question of "fast charging" and "normal charging", it is required to know that "Quick Charge" is basically a marketing terminology used for showcasing the power management capabilities of the circuitry in a smartphone and its wall charger. That implies that to make the "fast charging" work both the phone and the charger need to have the supporting technology for quick charging. But, that does not mean that you need a Qualcomm processor phone to avail that facility.
A quick charging can be supported in all the end devices that are contingent on the power controller alone and not dependent on a larger chipset. For that matter, Qualcomm has licensed the technology to non-Qualcomm smartphones as well. So, while purchasing an independent USB charger, make sure to check on these issues.

These days some of the phone companies are coming with two different types of USB Cables. One is Type C and another USB 2.0 broad connectors. Usually the tablets and bigger Note phones have those different broad connectors. So you have to check which type of connector applies in your case. If it's type C then any cable with the type C connectivity can be used.

Another point is fast charging. Your length of cable has nothing to do with fast charging. Usually if the connector wires are worn out or damaged inside, that can reduce the charging speed. So the length has nothing to do with how much time it takes for charging.

In the market there are USB cables available with different lengths. Getting large enough wired chargers for both broad and Type C connectors won't be harder. Amazon and other online sites have plenty of vendors who have different length cables for sale.

The USB cable provided to you by the company is best for charging your smartphone. According to law of physics, resistance is directly proportional to length hence longer the cable would be more time it will take to charge your phone. This can be managed only by using long USB cable of greater cross-sectional area. I should suggest you to use extension cable to compensate the shorter length of cable instead of using a longer cable.

The longer cable which you'll buy from the local market or online source will hamper the charging time of the phone and your phone will get slowly charged. You can check the difference yourself by installing charging applications on your phone that keep an eye on charging and discharging current.

One of the apps available on Play Store is "Amphere" app . The function of the application is to measure the charging/discharging current of your battery. When you'll not charge your mobile the discharging current will be shown as negative. When a charger is connected to your phone then some current is being consumed by the phone and the rest will be used to charge the battery. Let suppose that your phone consumes 200mA charge then when a 500 mA charger will be connected to your charger that charger would charge your phone with 300mA charge as 200mA will be used by the phone. There are many other apps available also you can get those apps available online.

It really doesn't cause any problem whether you use a long cable or a short cable. But sometimes while using other chargers, your mobile's rate of battery charging may slow down. I f you use a fast charger of a Moto Z or One plus, it sometimes increases the rate of battery charging.
But remember not to switch chargers on a daily basis. Your battery may get corrupt or it may not work.