The most economic and efficient way of storing around 20 TB of data is to buy an HDD and store it on that. Your best options are probably either a Seagate Exos X20 or a Western Digital Elements. Enterprise SSDs are possible options,but far more expensive than an HDD. You especially do not want to buy an Enterprise SSD, keep it offline and power it on occasionally.

Is NAS the perfect solution? Or should I use internal HDDs as the main storage and daily use? Thanks,

Depends a lot on your particular use-case. For most people OneDrive is a pretty good place to store personal files like documents.

I love the IBM x3650 range. My absolute favourite workhorse.

Desktop editions of Windows can be used for a simple home NAS. However, it doesn’t have a lot of advanced features supporting that use-case. Once you have a NAS, all of your digital data tends to end on it, which makes it a very critical system. Desktop Windows has one significant advantage, that you can run Backblaze personal computer unlimited backup on it to secure a backup of your critical and non-critical data in case of a disaster. On the down-side, there are no good RAID-features available for desktop Windows, making your data very vulnerable to drive failures, which are quite common. I personally prefer to run a homeNAS on something supporting the ZFS file system, such as Linux with OpenZFS or TrueNAS,but it is very important to choose a system based on your own skills, so you are able to set it up and manage it safely.

No. The storage industry is not a scam. HDDs are incredibly complex and delicate mechanical devices, that fail, despite manufacturers putting in enormous efforts to make them as reliable as possible.Portable HDDs are even more vulnerable, and you shouldn’t consider them long term investments. You should rconsider using portable HDDs as your primary storage, and set up a proper backup strategy.