Nvidia Optimus

Nvidia Optimus is an Nvidia technology that allows devices, mainly laptops, to have more than one graphics card and switch between them when required.
Example: Gaming notebook with an RTX 3070. When you are not gaming, there is no need for that graphics card to be working, using a lot of power and making noise and heat. So, Optimus will turn that GPU off and use your integrated graphics, which is much less powerful but also much less power hungry.
However, this technology has many disadvantages

1. It is present on pretty much every Nvidia notebook (AMD has a similar technology but I'm not aware of it's name) and on most of them you can't turn it off. Fortunately, pretty much all Lenovo Legions, including all AMD models, allow you to disable it.
2. iGPU is always enabled and requires small portion of RAM to be allocated for VRAM (1-4 Gigabytes) and often you can't change how much it will allocate
3. Sometimes video outputs are routed through the iGPU instead of the dGPU. That prevents you from having features like G-Sync (Lenovo gives you FreeSync when routed through an iGPU since both AMD and Intel support it). This is even worse with Thunderbolt equipped machine where the USB-C video output is almost exclusively routed through the iGPU. And when you disable Optimus (aka disable the iGPU), those video outputs become unusable.
4. It lowers performance. Since the dGPU is giving frames to the iGPU and that passes it to your monitor, performance can suffer, sometimes up to 20%. See this video from JarrodsTech. Disabling Optimus boosts your performance on integrated and external monitors.
5. If your HDMI, USB-C or DisplayPort ports are connected to a dGPU, you can bypass Optimus that way even if you don't have a Mux switch.

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MUX switch allows switching off the iGPU, but you need to restart the device. When the iGPU is off, the display output won’t be routed through the iGPU, so there will be no performance hit, and no RAM will be reserved for the iGPU. This isn't Nvidia exclusive as Lenovo showed on their all AMD RX6000 Legion

• Advanced optimus allows to bypass the iGPU, without having to have it disabled with a MUX switch, so no restart needed. The choice between iGPU and dGPU can be made while the laptop is running, and there will be no performance hit. In theory, the best solution, as it allows both the best battery life, as well as best gaming performance, at the click of a single button.
  
• In practice, the solution tends to be a bit buggy, which is why many users still prefer the solution with MUX switch, even though it requires a restart to switch between “best battery life” and “best gaming performance” options.
  
• Also on some laptops there is no way to fully disable Optimus or Advanced Optimus, so not having it can be a bonus to some users.

Disconnect any external monitors and open Nvidia Control Panel (the old, Windows XP looking one).
If the only section there is “3D Settings” then you have Optimus enabled.

In Nvidia Control Panel, look for “Configure Surround, PhysX”. There will be a diagram showing you what ports are wired to what. If you only see one GPU, your dGPU, you have Optimus disabled.

Assuming your laptop allows it, it will be either in your OEM's control panel or in your laptop's UEFI settings. There is no native Windows toggle.

— dustojnikhummer