Best way to upgrade laptop storage?

I've got HP Pavilion Gaming 15-dk0051wm but I can't find where to verify the empty slots (only answer I could find is chatgpt which says this laptop is quite flexible to upgrade).
And I don't know which option would be better external HDD or Internal HDD, I can't afford SSD and I have 256 GB Nvme in this laptop, so I think adding HDD would be better.

I've worked with PCs and done electrical work but no experience on laptop hardware.