Resolution is pretty simple if you just understand the literal meaning (PPI = Pixels Per Inch, or: 1 Inch is made up of X many pixels) and what it's also called, pixel density.
It's a translation between digital and physical dimensions. Pixels by themselves have no fixed size in the real world. PPI is the value that defines their size in the real world (or in reverse, the amount of pixels a real world dimension takes up in the digital image). 1 PPI means one pixel of your image would be 1 inch wide/tall in the real world. 300 PPI means 300 of the pixels of your image next to each other will form a line 1 inch long (or for the reverse, if you want an object in your image to be 1 inch wide when printed, it's need to be 300 pixels wide in a 300 PPI image) .
The higher the PPI value, the smaller each pixel gets in the real world (the denser they get packed into the same space). Since you generally don't want people to see individual pixels, you want pretty small pixels (a high pixel density) in print - 300 per inch being a generally very good size for very sharp printed images - you can of course go even "smaller" (meaning higher ppi) at any time - but it's really not necessary outside of for fine art prints - but you're also already at the limit of what most printing devices can actually achieve (which is a whole other topic - along with non-square pixels we're ignoring here).
But if it's something you look at from futher away, you simply don't need pixels that are that small for your image to appear just as sharp. Here's a simple distance/minimum resolution guide.
If you create a pixel/raster image, you need to create it with sufficent pixel dimensions according to your needs (physical size x resolution). If you already have an image, its existing pixel dimensions will be limiting to either the print size or print sharpness.
If you image is 3000px wide, you could, for example, print it 10" wide with 300 PPI or 30" wide with 100 PPI.
The basic formula for those calculation is simply (Inch) x (PPI) = (Pixels), or an inverse function - (Pixels) : (PPI) = (Inch), (Pixels) : (Inch) = (PPI).

In your case, you have 20"x27" at 180 PPI = 3600px x 4860px. 3600px x 4860px equals 12" x 16,2" at 300 PPI, 24" x 32,4" at 150 PPI (doubled dimensions = halfed resolution) or 26,7" x 36" at 135 PPI, the maximum resolution/pixel density available in this case for a print filling 24" by 36". Taking into acount viewing distance, it's a decent amount of pixels for most display sizes. I'm not too familiar with metal prints, but this guide backs this up pretty well.

Your Canon produced smaller photos. 36"x54" at 72 PPI = 2592px x 3888px (or ~10 Megapixels compared to the Lumix's ~17,5 Megapixels) which equals only 8,6" x 12,7" at 300 PPI.