All carmakers use plastic panel clips (also known as snap-in retainers) to hold door and splash panels in place. They’re easy to remove and reinsert if you use the right tool. If you don’t, you’ll destroy the fastener and possibly damage the panel. “Christmas tree” fasteners are the most common. You’ll need a door panel trim removal tool (available at any auto parts store for about $5) to remove them.
Rivet fasteners are also popular. They operate like a plastic drywall anchor; you push them into place and then they expand as you drive in either a screw or a plastic “rivet.” Remove the screw or rivet first. Then pull out the entire fastener.
You can find replacement fasteners at most auto parts stores for about 40¢ each. So don’t waste time trying to fabricate a new one out of nuts and bolts. Just take the old fastener with you and the store clerks will match it with a new one.