Hi Eveything,
Welcome to FP.
1) You will probably eventually get tired of working with the same limited palette of sounds (although there's plenty you can do with the basics). That is when having more sounds available comes in handy. A new sound might inspire you in a certain musical direction as well.
2) A sample is a short recording or snippet of sound that can be used in a number of ways in music/audio production (could be anything-- an instrument, a voice, a sound effect). They can be manipulated to sound like they are unrecognizably different from the original source sound (add effects, backwards, etc.), or can be "pure" samples. They can be used as "triggered" sounds (press a button/play a key = that sound), "one shots" (press a button/play a key = that sound plays through in its entirety), or as a "loop" (a start and end cue point are created, but the sample when triggered plays through continuously between those two points until stopped; this is useful for making simple drum tracks or using a vinyl sample as the basis for a beat production, for example). In today's DAW and workstation terminology, samples can have different names (loops, clips, sounds, etc.), but they all just really mean the same thing-- an available sound for you to work with in your production. In this way you can think of any sound you use as a "sample," whether it is from a tone bank on a virtual instrument, a hardware module, or a workstation/synth.
3) Sure, LMGTFY ("Let Me Google That For You"):
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https://www.steinberg.net/forums/viewtopic.php?t=78462
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https://www.youtube.com/watch?v=W0se3NDL72U
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https://www.youtube.com/watch?v=bKmDaq_sWNg
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https://www.techwalla.com/articles/how-to-add-vst-instruments-to-cubase
Again, welcome to FP and good fortune in your musical endeavors...