I think it's cool how sometimes stuff is standardized but there's variations in the realization of that standard, or that standard isn't fully enforced everywhere, or there's exceptions to a broader standard in specific contexts. It's hard to explain why this is so interesting to me but I find this sorta thing fascinating.
From watching old movies and shows, I think at some point (Columbo's from the 70s so I'll say at least during then) they were either blue letters on a white background, or yellow letters on a black or dark blue background, with all the writing embossed. "CALIFORNIA" was written in small capital letters on the top of the plate, and the general number format seemed to be 000 AAA. In newer plates, the default design has a white background with blue embossed letters for the plate's number, with red non-embossed letters at the top and bottom; the top says "California" in a cursive style, while the bottom has the DMV's website printed in lowercase letters; some plates with a similar style to the 70s ones still exist, but don't seem to be particularly common. Official vehicles have plates which say "CA EXEMPT" in printed red letters (can't remember if they were embossed or not). The most common number format seems to be 0AAA000, but I've also seen a few 00000A0's and 0000000's.
On standard plates the main format seems to be AAA A00, although I've seen a few plates with other formats; most of those still have two adjacent numbers with the four other characters being letters (00A AAA for example) but I've seen at least one with four numbers and two letters instead (000 0AA). There is a white field with a green outline and numbers, and a pair of oranges in the space between the numbers. There are a bunch of alternate plate designs that don't follow this pattern, I haven't seen enough of each to pick up on whether they do follow any pattern but some of them have only 5 digits. The motto below the plate number is usually "SUNSHINE STATE", but sometimes can be "IN GOD WE TRUST" or the name of a county (presumably the one the car is registered in). The plate number and motto are embossed, but the Florida name at the top (actually a link to some tourism website) is flat.
Since around 2015, these countries began switching to a unified license plate format. All of them have a blue stripe at the top containing the country name, the Mercosur/Mercosul logo on the left, and the country's flag on the right. Below the stripe is a white field containing the license plate numbers in Germany's license plate font (up until then only Uruguay had used that font); each country has a different numbering system. Brasil's plates have the portuguese Mercosul name, while the others have the spanish Mercosur. Despite this attempted standardization, there's some differences within the formatting of these shared elements:
During the 2000s and part of the 2010s, Argentine license plates had a black field with thick white outlines at the top and bottom, which became thin white outlines at the sides. The top white field said "ARGENTINA" in light blue, and had the Argentine coat of arms at its left. Within the black field were black numbers formatted AAA 000. If a plate had at some point been replaced due to being damaged or lost, there would be a small "D" in the middle (presumably for Dos/Doble/someothertypeof2). Rarely, if one of these plates was lost again, there would be a small T (Tres/Triple/other3) in that middle space. This plate format started getting replaced by the Mercosur plates in the second half of the 2010s; the number format changed to AA 000 AA.
Before the late 1990s/early 2000s, each department had its own license plate system. The departments were assigned a letter to represent them, starting with A for Canelones and assigning subsequent letters in a roughly counterclockwise inwards spiral (Maldonado B, Rocha C, Treintaytres D... ending in Tacuarembo T). The capital, Montevideo, did not have a letter. The earliest plates I've seen were pre-standardization Montevideo plates which followed the format 000 000; they had a white field with embossed black letters and text reading "MONTEVIDEO" under them. When license plates were standardized nationwide (with exceptions I'll describe later), the number format used was #AA 0000, where # is the letter assigned to that particular department; Montevideo was assigned S, the next available letter. The numbers now began using the German license plate font,the embossed text below said "URUGUAY", and a small image of the department's coat of arms was within the space. AA could be replaced by specific letters to represent special vehicle types, such as TX for taxis and OF for government vehicles; these would sometimes have extra text alongside "URUGUAY" and change the letters and background to a different colors. A small excpetion to this standard was Salto's license plates, which had one number less (HAA 000) and had a much larger coat of arms in the space. The much larger exception to the standardized plates was Maldonado, which for most vehicles didn't switch plate designs until the rollout of Mercosur plates. They were formatted B 000 000, with a large departmental coat of arms in the first space and the name of the town/region the plate was from in the text under the number. The font used and overall formatting was similar to Montevideo's pre-standardization plates. Mercosur plates were implemented during the second half of the 2010s, and by the 2020s have become the more common design. All plates now follow the #AA 0000 format, special use plates no longer change the background color but they can still change the color of the black lines/letters. Although Maldonado's license plates now fully follow the Mercosur license plate format, they also have an extra blue piece with white text attached to the bottom, containing the same town/region information that used to be on the plate.