Samus will unlock numerous abilities and upgrades throughout her journey in Metroid Dread. You will need these to traverse ZDR's different areas and overcome the enemies there. Samus' arm cannon in particular will receive multiple upgrades including the Plasma Beam.
The Plasma Beam upgrade can be found in Elun. You will start from the tram and sit through a cutscene. You will come across a green door dome that you cannot interact with. Nearby bomb the floor to open a pathway. Drop through and bomb through the next floor as well. You will find a Morph Ball Launcher—use it.
Climb to the top in the next room and enter the door on the right. Make your way to the bottom of this room and bomb the floor on the left. Head to the right and enter the door. This takes you to another room. Make your way to the bottom of this one too and you will find another Morph Ball Launcher.
Take the launcher and you will find a missile door. Blast it to enter the room with the Chozo Statue. Break the orb to unlock the Plasma Beam. With this, you can now destroy the green door domes that prevented you from progressing earlier. Your arm cannon also gets a damage boost and can damage metal enemies without the need to charge it up first.
For more information on Metroid Dread, check out How to get the Gravity Suit early in Metroid Dread – Sequence Breaking and How to Access Domed Doors in Metroid Dread on Pro Game Guides.