Check temps first. Then undervolt. If needed then repaste

Why don't get your own RAM and SSD since it's cheaper?

Model 81FV00M8MJ correct?

Will not void warranty (at least with my case). Changed HDD to SSD in my Y520 and service center still honor me warranty service. Even the guy told me that how it works: Laptop warranty still remains except for the SSD part (obviously)

It only supports PCIe/NVMe M.2 SSD like many Lenovo laptops. Dunno why they dun support SATA M.2

You can add 2.5 inch SATA SSD also

Yes.

Iinm, the 128gb ssd is 2240 size whereas 256gb and above is 2280 size. Which means its longer, and bigger in size. Some benefits are better memory block configurations as they are not cramped up and also better heat dissipation and heat generated much lesser.

But that is just on physical size alone.

Another diff is, the ssd model used itself. The 128gb is iinm by intel, its one of the slowest nvme ssd out there, akin to a sata ssd instead.. (but comparatively still way faster than hdd la).
I think around 1k+ read speed and a few hundred write speeds..

Compared to the 256gb we get Samsung PM981 model(one of the best nvme around) read speeds 3k+ write speeds 1k+. Stark difference in r/w speed.

Edit: you can check what components you have in your laptop by opening it up. Theres a tutorial on how to do it on youtube..

Or you can go lenovo support website enter your serial number (not model number) to get the exact config and components of your laptop.. from there you can also check the warranty details, download drivers, etc..

Union memory (China brand), probably DRAMless (since it's a 2242 M.2) and low quality

If the original one comes with 2242 (128GB) then yes. Remove the extension bracket and you can put in 2280

970 EVO is overkill for regular consumer

DRAMless not suitable for OS drive, get at least something like Kingston A1000

Best value probably A1000. Note that Legion series laptops don't support SATA M.2 (although Sandisk and Adata also got decent SATA SSD models)