I noticed a few inaccuracies in most of the comments under this answer so I wanna clear some things up, as someone who's actually personally looked at the real data:
- Eternatus, Zacian, and Zamazenta cannot Dynamax in any way, shape, or form, regardless of how you acquire them, be it by catching them, hacking, or using Mew or Ditto to Transform into it. The game has a specific flag for these three mons that specifically turns off the Dynamax button, even if Dynamax would normally be available (that is, you didn't Dynamax already in the current battle)
- Dynamax does not affect base stats in any way, and neither does Gigantamax. The HP increases as a direct function on the Pokemon's *actual* stats, that is, the stats you would see on the summary. At Dynamax Level 10 (a full bar), this multiplier is x2, so a Blissey with 680 HP would always Dynamax to 1,360 HP at DM Lv 10
- Eternamax is a bit different, and in theory doesn't have to be exclusive to Eternatus (for example, in theory, they could introduce an "Eternamax Zacian" if they wanted to, nothing about it makes it exclusive to Eternatus, except for the fact they just never gave it to anything else.) Eternamax is more like Mega Evolution, with actually unique base stats, which you can check on Eternatus's Pokedex Page: pokemondb.net/pokedex/eternatus
- Eternamax actually changes base stats themselves before applying the Dynamax multiplier to the HP. In Eternatus' case, the HP, Defense, and Special Defense Base Stats change.
- Because the HP is increased via a base stat change (to be clear: the ONLY dynamax that changes base stats at all) and THEN has the Dynamax multiplier applied, the HP goes a lot higher, but the limit on this is restricted by the same limits base stats already have, being unable to go above 255 because of how base stat data is stored.
