You do not need a new Windows license - you can download Windows 10 media from Microsoft.

If the system has an active Windows 10 install now (from the factory), the license code you need is already onboard.

You will need to reload Windows onto the new drive.

https://www.microsoft.com/en-us/software-download/windows10

And to add, do not take it back to that PC shop. Any qualified shop should know Windows 10 licensing procedures.

The active Windows install is on the existing hard drive.  The license key is on the system board.

You can either clone the existing install (as long is it will fit on the new drive) or install a fresh copy.

If what you've written is correct, the advice in your earlier reply is sound:  find a different repair shop.  This one is not competent.

If Windows is installed and activated on the existing drive, it will install and activate on any new drive you install.

No, a bitlocker key is not the Windows product key.  If the system came from Dell with Windows 10, it has a license key embedded into the system board.  If the system came from another source, that may or may not be the case.

If the system is encrypted with Bitlocker and you've lost the key, that's another matter entirely.  If that's the case, you will have no option other than to do a clean install onto a new drive or a complete partition-format-reinstall on the existing drive, assuming it has not failed.

You haven't been particularly forthcoming with the situation surrounding the need for a drive replacement, which makes it very difficult to recommend anything - since we don't know what the situation is, we cannot verify what you've been told.  It may be the shop is telling you something based on information you have not shared.

Just a little additional info.  As was mentioned, once that system is activated with Windows 10 that info is kept online.  If you buy a new drive and install, once you are online, it will reactivate.

Home Built Desktop systems behave the same way in that once you enter the License Key when you purchased Win 10, you no longer need it, you just skip entering the key with subsequent reinstalls.

The thing that does effect this is changes to your system and even then it depends on what was changed.  A new motherboard, a new license unless you have a retail key..  But authorized Repair facilities can move a License over on an OEM system in such cases.

You don't need the license key.  The installer will pick up the embedded key when you install Windows.

It will automatically activate when you first connect to the Internet after installing.