MacBook Pro 2012 (Non-Retina) not working! Loading with error symbols (prohibitory and folder symbols).

The only real weak spot in the non-retina MBPs is the hard drive CABLE. It is in reality a very thin and flexible printed circuit board with minimal electrical insulation. Its circuitous routing over sharp metal edges in the chassis make it prone to wear due to the vibrations of normal use and transport. This wear/damage usually cannot be detected with a visual inspection.

Thus, instructions for drive exchanges or conversions in one of that model more that 5 years old should be "ALWAYS REPLACE THE CABLE AT THE SAME TIME YOU CAHNGE THE DRIVE!"

The cables are cheap, available, and easy to install:

https://eshop.macsales.com/item/Apple/8211480/

Video instructions:

Hard Drive Cable Install

Based on product history alone, I would try that before anything else now that you know the SSD is working.

Try running the Apple Diagnostics to see if any hardware issues are detected. There is a possibility it may report an issue with the SSD since it is a third party SSD (does not necessarily mean it is the problem unless you ran the Apple Diagnostics previously and it passed).

Why is it taking so long to install Lion? Do you have a slow Internet connection for Internet Recovery Mode? If not, then it probably indicates a hardware issue with the laptop, possibly a bad or incompatible SSD or a bad hard drive SATA Cable which is very common especially on the 13" model when using an SSD.

You can try removing the SSD and connecting it externally using a USB3 to SATA Adapter, drive dock, or enclosure to see how the SSD boots & performs while connected externally. If this works fine, then it either indicates a defective internal hard drive SATA Cable, or a bad/incompatible SSD. You can try checking the health of the SSD by using DriveDx (free trial period) and posting the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper. Unfortunately drive monitoring apps like this do not accurately interpret SSD health since not all "failing" or "warning" conditions are fatal or even a problem.

FYI, the Samsung SSDs, especially the EVO model, has mixed results with some Macs for compatibility & reliability.

Try using Internet Recovery Mode using Command + Option + R to see if you can access the online macOS 10.15 installer. However, some Macs will still only boot into the online installer for the OS which originally shipped with a Mac. If you are going to reinstall macOS and you are only being given macOS 10.7 Lion as an option when using Internet Recovery Mode, then I would suggest creating a macOS 10.15 USB installer (or a USB installer for the OS you want to use which is compatible with the laptop).

What is the make & model of the other SSD you tried? And I assume it also had problems when installed internally?

Most likely the internal hard drive SATA Cable is bad especially if the second SSD is a different model. This cable has an extremely high rate of failure especially on the 13" 2012 model. OWC supplies them and has official resellers in various regions as well.

Otherwise it may just be a compatibility issue with the SSDs you are using. We've successfully used Crucial MX500 SSDs in these laptops (avoid the BX500 series) although Crucial may have changed them since the last time we bought them several years ago.

As HWTech point out, Samsung EVO models have a Bad reputation of being unreliable when used in Macs.