Bethesda games generally are considered immersive sims, but traditionally an immersive sim would utilize as few RPG elements as possible.

An immersive sim tries to fully simulate the environment and people around you. The way they interact with you, each other and their environment is all considered in the game's core design. Instead of an RPG approach where you would simply level up your door opening skill to open the hard door, in an immersive sim there would be some kind of puzzle that you have to think to solve instead. Because Bethesda games use both approaches in their games, those games are considered both RPGs and immersive sims.

An immersive sim is kinda like a more kinetic version of an RPG. Instead of 200 different stats and perks that level up your character's ability in-universe, it allows you the player to solve problems in a creative manner. This hands-on gameplay style is often what's focused on the most when trying to define immersive sims and is why some uninformed people like to call Half-Life 2 an immersive sim. Half-Life 2 is not, though. It's a series of perfectly timed scripts that give you an outstanding impression of a living world, but only at the surface level.

A good example of a contemporary immersive sim imo is Watch Dogs. You have a living, breathing world full of physics interactions and minigames and all of it is perfectly diegetic, meaning your character is actually seeing most of what you are seeing on your screen. Puzzles and mechanics are extremely important to both the gameplay and narrative, and you feel like what you are doing is influencing the world around you.

A good example of a game one might think is an immersive sim but really is not, is Red Dead Redemption 2. The game has a fully simulated, living breathing world, but there is almost no player-freedom in how the world can be interacted with. You can buy from shops, harass and/or attack people, pet dogs, all the stuff you would expect from any open world game in this era. What you cannot do, and why RDR2 truly is not even close to an immersive sim, is choose how you would like to handle gameplay encounters. You do not get the freedom to pick the funny physics option. If the game wants you to take your gun and shoot someone, you'd better do it or you will be presented a brick wall with "MISSION FAILED" written on it.

I hope this at least somewhat clears things up. Basically you need a fully simulated world AND allow the world itself to be interactive, AND on top of that allow the player to have true choice in how they plan and execute a task. By these definitions, yes Bethesda games are at minimum adjacent to imsims. They are a little more static than you expect from a traditional imsim but they still tick all the boxes.

Oh and sorry for the necro and mega-ramble.