These symbols are built in to the default ISO and ANSI templates provided by SolidWorks, and you can copy/paste them from there into your own template. Alternatively, simply draw your own - it's just a few lines and circles!
Right click in the drawing area, and then click "Edit Sheet Format", to allow you to select the lines you want to adjust
Open the Lines Toolbar - you can search for it if you don't have it visible
Change the line weight using the relevant tool
See the .gif below to show this process.
There's no difference at all - an extrude is an extrude is an extrude (unless it's a cut)
SolidWorks differs from some other CAD in that Boss and Cut are separate tools - others have just one extrude tool with a Boolean toggle in the feature manager
You can (and should for your own sanity) rename features in the tree to better represent what they are to you. ...
Yes, it's possible.
This is known as a hardware stack, and the official SW help pages provides a clear explanation regarding setting these up: https://help.solidworks.com/2019/english/SolidWorks/sldworks/c_Smart_Fasteners_Hardware_Stacks.htm#voc1450446632618