You’ve got the computer, so downloading is probably next.

You have to figure out how many SATA ports and drive bays each system has. A lot of Optiplex systems only have room for just 1 or 2 drives, sometimes just 1x2.5" and 1x3.5" with no room for two 3.5" drives which isn't exactly good for hoarding.

Consider getting a good external multibay enclosure. A DAS. Directly Attached Storage. If you share it on your local network you have a form of NAS. Network Attached Storage. You can access the contents of the DAS/NAS from any device in the network. TV, tablet, phone or other computers.

It is possible to pool all the drives to create a larger filesystem. But before you do that, especially before you consider RAID, fix the backup problem. HDDs can and will fail at any time. You can (and will) delete files by mistake. You protect against this by having more than one copy of everything. The harder to replace and the more valuable, the more copies you should have.

RAID means Redundant Array of Inexpensive Drives. It is a way to pool drives so that if one drive fail, the contents of the remaining drives in the pool still can be used to reconstruct what was on the missing drive. I don't use RAID. RAID is great for businesses that need to stay online 24/7. They still need backups. I think the most common reason for data loss is user error. You simply delete something when you thought you were doing something else. RAID provides no protection against that. Backups do.

Consider using only very large HDDs. The largest you can afford. Use old smaller drives for extra backups or get rid of them. That way you don't need as many drive bays, use less power and makes less noise. Multibay enclosures are great because they reduce the clutter of cables.

If you have two 20TB drives in an external enclosure, you can use one drive for storage and the other for backups of the first. Later, when the first DAS is full, get two DAS. One for storage and one for backups of the first.

Look up 3-2-1 backups.

You might consider buying a second hand small, low power, office PC and use it as a headless (without monitor, keyboard and mouse) server, connected to the DAS.

I only use Linux.