How I can use Desktop CPU's 500 GB harddisk as external storage and what device I would need to ru

Of course, you can connect your hard disk/SSD to an Android phone. To do that you need an OTG cable. But make sure your phone supports OTG connectivity. If not, you can still connect the HDD to any other laptop or desktop using an HDD SATA cable and copy the data. Later, you can copy the duplicate files to your phone using a USB cable.
The former method is an easier one compared to the latter one.
A typical 1TB external Harddisk cost is around 5000INR.

The internal hard disk in a desktop PC is a dedicated item for that model of the PC in the sense that there will be some power supply and connecting bunches to it. If we want to use it as an external storage disk with a phone or other computer then the connecting bunches would be an issue as there should be compatible connectors available in the market. It is difficult to say whether the available ones will resolve the issue.
I would suggest to have an opinion of the computer repair technicians who can make some arrangement of connectors from some old PC and then use them for the purpose. There are no readymade standard connectors available in the market for such use of old internal drive of a desktop computer for external use.

It is possible to use an SSD or HDD as storage with an Android smartphone, but it requires some additional hardware and setup. Here are the general steps to follow:

1. First, you will need to purchase an external enclosure for the SSD or HDD that is compatible with your smartphone. There are many options available, and you should make sure to choose one that is compatible with your smartphone's USB port.

2. Once you have the enclosure, you will need to insert the SSD or HDD into the enclosure and connect it to your smartphone using a USB cable.

3. When you first connect the drive to your smartphone, you may need to format it in order to use it. You can do this by going to "Settings > Storage > Format as internal storage." Keep in mind that formatting the drive will erase all data on it, so you must make sure to back-up/copy-store all your important data first.

4. After formatting the drive, you should be able to use it as additional storage for your smartphone. You can move files and apps to the drive to free up space on your smartphone's internal storage.

Make sure that using an SSD or HDD as storage with an Android smartphone may not be as fast as using the smartphone's internal storage, and it may also require more power, which can drain the battery faster. Moreover, if you disconnect the drive from your smartphone, any apps or data stored on it will not be accessible until you reconnect the drive.

My personal suggestion would be to use a cloud service for storage since that will also protect you from any physical damage to your storage devices and will keep your data available to you anytime from anywhere via Internet.