You can merge Outlook PST files with a free manual method. To use the manual method, you will need MS Outlook, only then you can do it manually.

There are Manual Steps to Merge the Outlook PST file

Open Outlook:

Start Microsoft Outlook on your PC.
Making a New PST File:
Navigate to the "File" menu.
Click "New Items," then "More Items" > "Outlook Data File."
Select "Office Outlook Personal Folders File (.pst)" and then press "OK."
Give the new PST file a name, choose a location for it to be saved, and provide a password if necessary.
Select "OK."
Step 2: PST Files Should Be Imported Into a New PST

Go to the Home Tab: In the Outlook ribbon, click the "Home" tab.
Launch the Import Wizard:

Select "File."
Select "Open & Export" and then "Import/Export."
Choose Import from a Different Program or File:
Select "Import from another program or file" and then press "Next."
Select a File Type:
Choose "Outlook Data File (.pst)" and press "Next."
Select PST file
Click "Browse" and navigate to the PST file location you want to import.
Select the file and press "Next."
Select a Location:

As the destination, choose the freshly produced PST file.
Select how you want duplicates to be handled.
Select "Next."
Finish the import:

To begin the import procedure, click the "Finish" button.
Repeat these instructions for each PST file that needs to be merged.
Step 3: Data Organization and Verification

Go to the left navigation pane in Outlook and organize the folders and items in the newly created PST file.
Check the Merge:
Examine the new PST file to ensure that all items from the imported PST files are present.
Step 4: Consolidate the New PST File
PST File Compression:
Right-click on the new PST and select Properties.
Select the "General" option and then "Compact Now."

The manual method is risky and will waste your time. Merge PST files with a professional tool. I suggest utilizing the BetaVare Merge PST Tool. This application is the most secure and bulk merge Outlook PST file. It easily merges multiple Outlook PST files with password protection into a single Outlook PST. It supports all versions of Windows OS and Microsoft Outlook. You can download a free trial version of this application to test its features and usability before purchasing it.