[RPG] Which modifier shown in the D&D Beyond character sheet do I add to attack rolls with the longbow

Yes, you're understanding it correctly.

You roll your attack roll for the bow with proficiency and Dex modifier (PHB p. 194), and damage with just Dex (PHB p. 196).

Kinetic blasts are described as ranged attacks, and unless otherwise noted you add your Dexterity modifier to ranged attack rolls

From the Kineticist page :

Physical blasts are ranged attacks that deal an amount of damage equal to 1d6+1 + the kineticist's Constitution modifier, increasing by 1d6+1 for every 2 kineticist levels beyond 1st. Spell resistance doesn't apply against physical blasts. Energy blasts are ranged touch attacks that deal an amount of damage equal to 1d6 + 1/2 the kineticist's Constitution modifier, increasing by 1d6 for every 2 kineticist levels beyond 1st.

From the general combat rules :

Attack Bonus

Your attack bonus with a melee weapon is : Base attack bonus + Strength modifier + size modifier

Your attack bonus with a ranged weapon is : Base attack bonus + Dexterity modifier + size modifier + range penalty

Touch Attacks

Some attacks completely disregard armor, including shields and natural armor—the aggressor need only touch a foe for such an attack to take full effect. In these cases, the attacker makes a touch attack roll (either ranged or melee).

So you just use your Dexterity modifier for attack rolls.