Conversion
In short: Apex Legends sens times 3.8397. 1.2 becomes 5. For your own value, just edit the sentence below.
In the game you find the setting under Options → Controls → Mouse → Horizontal and Vertical Mouse Sensitivity.
The in-game slider runs from 1 to 100 in whole steps. Set horizontal and vertical sensitivity to the same value. The in-game field of view is vertical; in the converter enter the horizontal value (90 vertical is about 121 horizontal). The result is rounded to a whole number, so your mouse distance differs slightly.
Apex Legends turns 3.84 times further per mouse count than Rainbow Six Siege. That is why the number gets bigger while your mouse distance stays the same.
Rainbow Six Siege shows 14° less field of view than Apex Legends by default (91.49° against 105.5°). The picture looks a little tighter and movement on screen a little faster. For Apex Legends the field of view is only a rough guide, check it in the game.
In Rainbow Six Siege the slider only takes whole numbers. The result is rounded, so your mouse distance differs slightly.
Multiply your Apex Legends sens by 3.8397. 1.2 in Apex Legends becomes 5 in Rainbow Six Siege.
Options → Controls → Mouse → Horizontal and Vertical Mouse Sensitivity. The in-game slider runs from 1 to 100 in whole steps. Set horizontal and vertical sensitivity to the same value. The in-game field of view is vertical; in the converter enter the horizontal value (90 vertical is about 121 horizontal). The result is rounded to a whole number, so your mouse distance differs slightly.
Apex Legends: Slider from 70 to 110 (about 105 to 106° horizontal at 90). Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Match the field of view as closely as you can; then the converted sens feels most natural.
No. The values are for hipfire. Zoom and scope sensitivities are set separately in the game.
No. Keep your DPI and only change the sens in the game. That keeps your feel the same everywhere.