Here’s the simplest way I’ve fond to combine multiple PDFs into a single PDF. Here are the steps:
First Step:
Arrange your PDFs in a folder. Prefix them with numbers and sort them sequentially, and select the first one:

Second Step:
Press command+a to select all the files in the folder:

Third Step:
Right-click the first file in the listing and choose Quick Actions > Create PDF:

Fourth Step:
The new, combined PDF will be created at the top of the file listing. It’s name will be based on the name of the first file in the listing (with a space and “2” appended).

Fifth Step:
The file will be selected, waiting for you to rename it. You should do so. Here, for simplicity, I’ve named it “Combined.”

Conclusion:
That’s it. Hope you find it useful.