You're letting mechanical symptoms overwhelm you.
Turn on headlights. Are they dim?
With headlights on, turn key to start. What happens to the headlights?
Do they dim or do they click off when the key is turned to start? Do you hear a click? Or silence?
Clean the engine starter battery terminals, whether you think it's the problem or not. Are the battery posts shiny? If they are black is the coating soft or hard and crystalline?
Starter motors can fail in 99 ways and every last way is common.
It is impossible to "mental" your way through this. Start by jumping the battery from another vehicle and let the other vehicle charge your battery for ten minutes before turning your key again.