What happens when a laptop gets a new hard drive installed?

I know that the files are gone.
What about the function? (Are the programs running much better?)
What is the hard drive doing? (What are the functions of such a part?)

It runs the same as always, unless it doesn't have an SSD yet.

All data is stored persistently on the hard disk. It starts with the operating system and ends with the user's data (files, user data, etc.).

That answers the question about the function.

It depends on whether programs run "much better". If a small and slow HDD was previously in it, "the programs" will run noticeably faster if a larger and faster SSD is installed. If you do it the other way round, then it's just not like that.

The files do not have to be gone when you transfer them from the previous mass storage device to the new one.
Whether the programs run better depends on the speed difference between the old and the new mass storage device.
It should save everything that should not be lost when the computer is switched off. These are, for example, files for images, programs or other data.

The hard drive stores almost everything that you work with in the PC, programs, data, documents, pictures and games and settings.

Depending on how it is installed, some or all or nothing will be lost.

a) If the new plate is also installed, nothing is lost. You can then copy everything from the old to the new disk and expand the old one.

b) If only the new one is installed and the old one is removed beforehand because only a single disk fits in, of course everything is lost. In order to keep something, you could temporarily save the contents on an external disk before the conversion.

C) since you can't answer such a simple question yourself, I wouldn't do it myself.

I know that the files are gone.

that is already well recognized. However, if the old disk is still partially legible, the data can be saved quite easily. Depending on the situation, a professional data recovery may be worthwhile.

What about the function? (Are the programs running much better?)

yes and no. The programs generally run better on a freshly installed system. And if the plate has already shown signs of wear, there's also a slight improvement. I would be a little careful with that much.

What is the hard drive doing? (What are the functions of such a part?)

the hard drive is the long-term memory. Not only the programs, but also your own files such as images, texts, etc. Are usually found on the hard drive. (You can also save them externally, e.g. In an internet cloud or on a usb stick, etc. In addition, all the settings such as the background of the desktop, the drivers for the graphics card, etc. Everything that has to be saved when the device is off.

A small note still SSD (ie chip-based hard disks, which are suitable like a USB stick etc. Funzen are a very good choice for such a laptop. The drives (if you want to call them that) no longer cost the world and just have to operate a few advantages in the laptop.

you need less electricity
they are shockproof

Apart from that, because the physical gathering of files is no longer necessary, an ssd is much faster than a normal hard drive.

Whether the PC is faster depends on whether the new disk is faster than the old one, e.g. SSD instead of HDD. If you do not reinstall the system, the programs will not run faster with the same performance data on the new disk, only because of the replacement.