Budget 90k+ Gigabyte RTX 3060 12gb vs Gigabyte RTX 3060ti?


3060 ti is better. The purpose of VRAM is to load the entire data from a scene so that the GPU can process it and show you the frames. VRAM is the RAM of a GPU. Right now 8GB VRAM is more than enough upto 1440p.
Gigabyte has multiple 3060 ti models. I recommend you to go for Gigabyte tripple fan 3060 ti if you have enough GPU clearance in your case. I had a Gigabyte tripple fan 3060 12 GB for a few days and it was barely audible even on load.


And whats the purpose of gpu built in ram
Processors need data to be in RAM so that they can access and work on it fast. Processors are very fast and directly working on data from hard disk or SSD will make them extremely slow as they will spend most of their time waiting for data to arrive. For this reason, each processor needs access to RAM.

Consider this simple analogy. Let's say you are making a dish in your kitchen and you need a total of 12 ingredients at different points while making the dish. Every time you need an ingredient, you go rummaging through your shelf and/or refrigerator trying to find it. This will make the whole cooking process slow and tedious. Instead, what you could do is gather all the ingredients and lay them out in front of you, then start cooking. This way whenever you need something, it's readily available in front of you. This makes the whole cooking process go smoothly. Let's say you were also working with an assistant and while you are making one dish, this assistant makes sure to keep the ingredients ready for the next dish. This arrangement will make you far more efficient. Keeping the ingredients ready is akin to keeping the data in memory, readily available when needed.

Graphics card is also a processor, just a dedicated processor only for graphics (and some ancillary) processing, while the computer's main processor (say, Core i5 or Ryzen 5600) are general purpose processors. This is why graphics cards are also referred to as GPU or Graphics Processing Unit.

There are two reasons for the GPU to have dedicated RAM (and don't use the RAM already there in the system):
  1. Due to the nature of processing that it does, GPUs need a slightly different kind of RAM and the regular RAM is not a good fit for it
  2. Due to the RAM being on the motherboard, GPU will not be able to access it fast enough.
For these two reasons, having dedicated RAM on the graphics card makes it more efficient.

Note: While people use Graphics Card and GPU interchangeably, they are not the same. A GPU is the graphics processor. A Graphics Card is a collection of components which include the GPU and the dedicated RAM, among other things.