Unless it specifies "racial hit dice", use the sum of the creature's racial HD, its class levels (or even "bonus HD" in case of things like Animal Companions) to determine the creature's total number of HD.
This definition is somewhat scattered, but see
Hit Dice: The term "Hit Dice" is used synonymously with "character levels" for effects that affect a number of Hit Dice of creatures. Creatures with Hit Dice only from their race, not from classes, have character levels equal to their Hit Dice.
on page 181 of the Player's Handbook.
Hit Die/Dice (HD): In the singular form, a die rolled to generate
hit points. In the plural form, a measure of relative power that is synonymous with character level for the sake of spells, magic items, and magical effects that affect a certain number of Hit Dice of creatures.
on page 309 of the Player's Handbook.
Much of it is also inferred from usage, such as in monster entries (Look at the Troll Hunter, Monster Manual page 247).
"Always" isn't precisely the right term
A creature that is "always" a certain alignment is only that alignment about 95% of the time, with the other 5% being composed of various exceptions such as those created by spells, an inclination to a different alignment, corruption, redemption, or just plain cussedness.
Alignment is contradictory
Though many sources (the Player's Handbook, Book of Exalted Deeds, Book of Vile Darkness, Heroes of Horror, Champions of Valor and Champions of Ruin especially) talk about alignment, they don't particularly get along with each other. Alignment is a contradictory mess full of double-standards, exceptions, confusing 'hard rules', and just plain lack of moral thought. This makes it hard to talk about in 'always' cases, especially ones like Half-Celestial that were published early on.
However, free will is a common trend
Think of Half-Celestials as having an innate instinct for goodness - they understand about mercy, empathy, compassion, love, and forgiveness the way normal people understand about breathing. As free-willed beings, they don't have to listen to these instincts - but they do have to live with them. Evil is not comfortable for such beings. Perhaps some turn to it out of a perverse thrill, the way masochists get attached to pain, but outside of those exceptional few evil is difficult and uncomfortable for beings whose essence is at least partially composed of elemental goodness the way a fire elemental is made of fire. No, there's nothing stopping a fire elemental from taking up, say, water skiing but it's certainly not the best hobby in the world for it.
There are examples of 'always' beings outside of those alignments
Fallen celestials, lawful slaadi, chaotic modrons and risen fiends exist in the 3.5 universe, often as published NPCs. The alignment rules state that if a being acts in accordance with a new alignment, they take up that alignment. From a mechanical perspective, if a half-celestial does consistent evil, then it is evil-aligned.
Best Answer
Lower AC is better for that game's D&D version
That video game (in your screenshot) and table derived from it are based on Advanced Dungeons & Dragons, 2nd edition – which uses the THAC0 ("To Hit Armor Class 0") system, in which lower armor class values are better (and negative AC values are both possible and even better than low positive values).
In the current (5th) edition of D&D, and in each of the WotC-published editions, a higher value is better since they are based on the d20 System. Your link to Roll20.net is to the 5th-edition D&D rules, and that uses a "higher is better" system. That change, from TSR to WoTC, happened starting with the 3rd edition of Dungeons & Dragons, published in 2000. (WoTC bought out TSR in 1997.)
Not all D&D versions are the same. You have to read the label. 😊
Having a high Dexterity, in 2nd-edition AD&D, makes increasingly useful Armor Class adjustments (-2, -3, -4) as the Dexterity score gets higher.