Pokemon Legends: Arceus introduced itself in the February 26th Pokemon Presents as a prequel to Pokemon Diamond and Pearl, and it revolves around the creation of the very first Sinnoh Pokedex. Players will seemingly run across a fully open world for the first time in Pokemon history to catalog everything Sinnoh has to offer. Because the game takes place so far back in Sinnoh's history, the region's usual starter Pokemon --Turtwig, Piplup, and Chimchar-- haven't been discovered yet.
As a result, the player gets to choose three other Pokemon in Pokemon Legends: Arceus. They aren't completely new Pokemon, but two of them never appeared in the original gen-four roster. These three Pokemon include the gen seven grass-starter Rowlet, the gen two fire-starter Cyndaquil, and the gen five water-starter Oshawott. These are the Pokemon trainers will use to discover the original Sinnoh trio, who will then go on to become the new starter Pokemon for future trainers.
Of course, none of these Pokemon are native to Sinnoh, so they get to the region by way of the Pokemon professor that trainers receive the Pokedex from. This professor caught them while conducting research in their respective regions, those being Johto for Cyndaquil, Unova for Oshawott, and Alola for Rowlet. The Pokemon Legends: Arceus breakdown implies that the professor arrives in the one town of note in the region and recruits the player in their mission from there.
While Pokemon Legends: Arceus uses Pokemon from different regions and generations as its starters, it's unlikely that players will be able to catch anything outside of the Sinnoh region. Again, the game revolves around the creation of the Sinnoh Pokedex, and with Pokemon Legends: Arceus being the first true open-world game of the franchise, it's unlikely that it will let players explore different places in the hopes of completing something like a national dex.
All of that said, if Game Freak decides to apply the DLC concept it started in Pokemon Sword and Shield to the game, then it's certainly possible. The developer claimed it wasn't doing any more sequels or updated games anymore, so if it wanted to continue on the open-world concept — and there's no reason why it wouldn't — DLC would be the way to do it. It's also fairly likely that Pokemon Legends: Arceus will interact with the newly announced Pokemon Brilliant Diamond and Shining Pearl remakes, and they'll include Pokemon from every generation so far.
Pokemon Legends: Arceus releases in 2022 for the Nintendo Switch.