The database ( DB ) structure often changes between versions. The implication is that, for example, one cannot import a 2005 database into a current install and expect it to work. You need to install your original version of WordPress, import your db dump and then start upgrading.
If you plan on upgrading across multiple versions, follow these instructions: https://projectdmc.org/support/article/upgrading-wordpress-extended-instructions/
So… I would need to download a 2005ish version of WordPress… and then set up a local webserver on my machine to install it? (since nowhere online will let me install wordpress from 2005).
According to https://projectdmc.org/documentation/article/wordpress-versions/, this should be a WordPress 1.5. You can find its releases here: https://projectdmc.org/download/releases/#branch-15
Unfortunately, I cannot find any information on which PHP and database version such an old version requires. My recommendation would be to set up a local virtual machine with a Linux distribution from this year. For example, an image of Debian Sarge as a basis would be possible: https://www.debian.org/releases/sarge/
Of course, you will not find a hoster that provides such old software, so the local way would be strongly recommended.
If you are unable to cope with the above, find someone who can help you. You can find someone here, for example: https://jobs.wordpress.net
And finally, why do you want to do this? Is it about contributions from a web from that time? Then it might be worth considering importing the relevant data with an individual script. To do this, you would have to install an up-to-date WordPress (which is easy to do), import the backup into a second database parallel to the WordPress database and then develop an individual PHP script that accesses the second database and imports the data into the current version. Again, you should look for someone who can help you with this if it is not possible for you.