I have read tech patents available on Google. I wonder how patents can be avoided to be reinvented or understood so neatly and specifically given a patent file is usually tens of pages written very strictly. Everyday thousands of tech patents are filed and approved with millions already there, so how can somebody comprehend those and avoid reinventing an already invented claim? Please reply thoroughly!
If you are overwhelmed by the vast number of patents, there is not much of a cure. On the other hand if you have a specific invention you have made and want to find out if it is patentable, you can do a patentablity search - another answer from Eric Shain outlines how you could go about it. I have experienced a phenomena of dead-on prior art existing but not being found by the inventor or anyone else who wishes that the prior art didn't exist or who assumes it doesn't exist. It is hard to find something you are wishing/assuming you don't find. To address this and other difficulties of searching there are search professionals with fairly reasonable fees -if you have an exact target invention.
I have used National Patent Services nationalpatentservices.com. They and other patent searching companies employ people who spend their whole day at the public search room at the USPTO office in Alexandria VA using the same system USPTO examiners use. They generally are organized, like patent examiners, by technology. It might cost a few hundred dollars for a search - but you need to have a specific and succinct definition of what you are searching. Another option is Quick Patents quickpatents.com - they quote $399 for a patentablity search on their website and I have heard they do a good job. About 8 years ago I and Kevin Prince, the owner, were fellow board members of an inventor's club.
The short answer is you perform a patent search. You are absolutely correct to suggest this is a substantial undertaking. I am not a professional patent searcher, but I've done my fair share. You can certainly start with Google Patents, but I've found The Lens to have some useful additional features. My process is generally to try different search terms to narrow the number of hits to a manageable number and briefly read the patents focussing primarily on the initial independent claims. If I find a relevant patent, I then look at the cited patents. Iterate as necessary. I usually start with US patents to reduce the numbers a bit but you will eventually have to search internationally. Professional searchers also use patent classification codes. If you find a number of relevant patents then look for common classification codes.