I look for all these videos on youtube of projects on SolidWorks and I wonder how they get these accurate dimensions I want to make I project too without searching on Google for dimensions I want to make these dimensions
You are sort of out of luck unless you create a time machine.
With a time machine, you could travel back before standards were adopted, and everything manufactured was a "good guess" for a fit. How many shafts were made for the first steam engine before someone started to figure out exactly how much clearance was needed on a rotating or reciprocating shaft? Now we have standard tables for clearances.
If you use standard parts (nuts, bolts, screws, pins, shafts, drills...etc.) you'll need to look up the dimensions and make sure they will fit your product.
If you are designing other things, like say a wrought iron gate, then the choices you make will still be somewhat dictated by aesthetics. If you have a "good eye", or are a great artist, you may be able to make it up as you go. Otherwise, you'll want to stick to fractional proportions which have been in use since the time of the Greek empire (maybe earlier?).
What sort of things are you looking to design? Machines are generally built for a purpose, and use stock parts to keep the cost low.
Mechanical parts are designed to be cost effectively mass produced.
Once 3D printing gets better/cheaper, designing for 3D printing as the manufacturing method will allow for some really great blends of art and science.
Until then, I think you'll need to look into designing consumer products to be able to use more artistic talents in the design process. Consumer products are often designed to look good, instead of being simply functional.
option 1: use content center to find the things you need fast
option 2: make a library of standard parts your gonna use often (parts your not gonna change mid project)
option 3: make dynamic parts where you can alter a few values to result a new part (using excel can make this more easy as you can copy manufacture dymensions into the excel and then link it to your part)