Help with screws. There are many varieties lol

Hi all. Sorry to ask sucha basic question but I don't want to damage something by using the wrong screws in it. I'm in the process of moving everything to a new case, and it came with a few diffeent types of screws. I'm pretty sure the shorter ones go in the PSU. I'd mainly like to know which to use with the hard drives, because this case came without a bay and I need to screw the hard drive into the case. Thanks for any advice.

Ps: I think the fact that I'm mobile is not letting upload pics. Will upload in a minute.

Well there are quite a number of varieties, and maybe someone here is good enough to pick them out from simply seeing a picture. If we wanted to be fully scientific about it I suppose you could post a pic of the screw next to a mm rule and we could compare pitch. I can tell you the one on the left is usually used to secure IO brackets and blanks for PCIe slots/cards. The finer pitch one on the right does look like a motherboard mounting screw. I believe the one on the left would also be used for the PSU. I'm really not great at remembering which screw goes where though

But if you want to know what I do, not saying it's best practice but its been fine for me, is to try to thread it in by hand. If it works easily then you know its the correct diameter and pitch. If not then don't force it. You still need to make sure the length is appropriate for your application. When I first installed my current build into the case, I initially used the incorrect motherboard screws, but this was apparent because they bottomed out before the motherboard was completely secure.

General rule with any PC building, is don't force anything. Ever. It's all designed to go together easily and in only one way (odd exception - 4 pin RGB headers, why these aren't keyed is beyond me). Also screws don't need to be torqued down. Snug is fine. Same goes for spring mounted coolers, snug is fine. Really the main reason you're using a screwdriver is because it's small and awkward, not because you need the mechanical advantage.

the one on the left is for the psu, middle looks to be for a cd rom, mechanical hard drive, has the flatter head on it, the right one looks like a finer thread and to me is a ssd drive screw, poke a toothpick gently in the holes to see how much depth you have so to make sure its not going to hit anything, screw the screws in by hand first to make sure the thread is right and matches the screws, then when fitted nip them up with a screw driver

left psu/io ports, middle drives 3.5" and 5.25", right is for mobo

The one on the left is known as a tri-lobe screw. It is a type of sheet metal screw and is designed to fasten items that have no threads in the items being attached as it deforms the metal's holes; it is a case-hardened screw with a surface hardness of Rc 60. The two screws to the right of it are both machine screws. The center one is coarse thread, the right one is fine thread, different diameters as well. They could be either metric (most likely) or imperial standard depending on where the product is designed or intended for use. They are both soft, i.e., with a hardness of Rc 20-22 and are designed for attaching items that are pre-tapped with threads matching the screw's configuration. Thread guages are available for measuring both the diameter and pitch which is the best way of determining their size, etc., but as said above, try running the machine screws into the threads in the tapped item and if it goes in easily you've got a match, otherwise, no.