When should I cut fabric with scissors, rotary cutter, or knife?
It's mostly a question of personal preferences and boils down to a mix of what shapes you need to cut and your bodily capabilities.
Shapes
If you need to cut many straight lines, gentle curves or use a hard template to cut out many pieces, most people prefer a rotary cutter for its speed and capability to cut around the edges of a template without having to mark the outline first.

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Another advantage of a rotary cutter is that it can cut through multiple layers of fabric at once without the pieces shifting out of place. This is often used by quilters.
A rotary cutter produces the cleanest cuts if you can cut the whole outline (or at least one entire side) in one go. That makes it impractical for very large pieces like shades, sails, upholstery and the like.
A rotary cutter also fails at notches.
For straight cuts some people prefer a rotary cutter with a long ruler, others fold the fabric over and cut along the fold with straight scissors. I once saw a video where a woman explained that the reason why Japanese fabric shears look different than western ones is that you only need to cut straight lines to sew a Kimono. The fabric is folded over where you need to cut and one blade of the shears is inserted into the fold. By gently pulling the fold taut you create a straight cut but you still have very much control over the cut due to the shape of the shears.

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I honestly never heard of cutting fabric with a knife (at least not in a professional context). I assume you could do that for straight cuts when the fabric is folded over, but I'm afraid the risk of a crooked cut or snagged thread is much higher than when using Japanese style shears.
Capabilities
This is a very diverse topic and always depends on the individual user of the tool.
There are for example left-handed people who were forced to use scissors with their right hand in childhood and now left-handed scissors feel awkward to them. They might prefer a rotary cutter for its simplicity.
On the other hand, you need the strength to push the rotary cutter forward and through the fabric. There are people with physical disabilities that do not have the strength or are unable to stand up and lean over the fabric to cut it. Most of them prefer scissors.
There are also scissors with a spring that opens the arms after each cut, which makes them easier to use for people with disabilities or muscle weakness.

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And lastly there are people with impaired vision that might need to hold objects very close to be able to see them sharply. They may not be able to use a rotary cutter for bigger pieces because the edges may lay outside their range of vision. By marking the fabric and using scissors they can pull the fabric close enough to see clearly where they cut.
Price
Both tools come in a great range of prices, but one set of good quality fabric scissors can serve you for years while a rotary cutter needs replacement blades and a cutting mat. The mat must be at least as big as the biggest piece you need to cut, so bigger is better (but also more expensive).