it’s the alcohol. lots of (single) ipa have significantly higher hop bills than double and triple. the original gravity and yeast sets the ABV. many doubles are smoother in mouthfeel than singles, although run the risk of having a boozy aroma.
and bitterness is related to hop content, but specifically alpha-acid hop additions, so it can be controlled independently of the aroma hops.
secondary fermentation is common in many styles, other than ipa, and doesn’t typically have a huge influence on alcohol content, but rather allows flavors to mature, or become more pronounced. more yeast falls out of solution, for a clearer beer, and it can naturally carbonate.
some yeast strains will cause extra fermentation as time goes on, including when it’s bottled or aged in a cask, but that is not the case for most commercially available ipa.
and if you really want a smooth, drinkable, high alcohol beer, try finding a Baltic porter!
siqq! has a very “dead cells” style