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Sweat cools you by evaporating, not by appearing. Outdoors, moving air evaporates it almost as fast as you produce it. Indoors it has nowhere to go, so it sits on your skin and drips off without cooling you, and your body responds by producing even more. Airflow is the whole answer.
This is the part most people have never been told: sweat only cools you at the moment it evaporates. Sweat that rolls off your arm and hits the floor has done nothing for you at all except cost you fluid.
Outdoors you barely notice sweating at speed, because the air is taking it away the instant it appears. Indoors, with still air, the layer of humid air sitting against your skin stops evaporation. Your temperature keeps rising, so your body keeps calling for more sweat, and you end up with a puddle and no cooling.
That is why an indoor session can leave a pool under the bike when the same effort outdoors leaves you looking fine.
A fan does not just make you more comfortable. It makes your sweat work.
Moving air strips away the humid layer against your skin and lets evaporation happen, which is the mechanism that actually cools you. The practical effects are a lower heart rate at the same power, less fluid lost, and a session that does not fall apart in the last twenty minutes.
Aim the air at your head, face and chest rather than your legs. Your legs are not where you lose the most heat and they are not where discomfort starts.
You will lose much more fluid indoors than on an equivalent outdoor ride, and you will not feel thirsty in time to keep up.
Sweat is salty and it corrodes. It gets into the headset, the stem bolts, the bar tape and the top tube, and it causes real damage over a winter.
A towel over the bars and top tube, or a purpose-made sweat guard, plus a mat underneath, deals with almost all of it. Wipe the frame down afterwards rather than leaving it to dry. See protecting your bike from sweat damage.
No. If anything, trained people often start sweating sooner and sweat more, because their bodies are quicker to start cooling. How much you sweat is a poor guide to fitness.
Not necessarily, and that is fine. What tends to improve is how well you cope with the heat, not how little you produce.
Yes, if the room is closed up. A cold room with no airflow still stops evaporation. Opening a window often helps more than turning the heating down.
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