Canadian Quarters With The P

First of all-
to the community
Feel free to ask any question you may have as there is someone here who can help.

To answer your questions-
The "P" on the quarters stands for plated to signify the new multi-plated coins issued for circulation between 2001 and 2006 so there is no extra value.

The California Quarter is one a series known as the "State Series" issued between 1999-2008 when there was a different issue for each of the 50 states. As there was between 500 million and 1 billion of each state minted, there is
no extra value for these in circulated condition.

Your 1807 Great Britain George III penny may have some value depending on the condition.

There are a few 2000 P caribou and creativity 25 cents out there and are worth a small fortune

to the Community!

as mentioned previously, 'P' indicates "plated steel".

When the mint was going away from solid core blanks, in many instances they would do part of the yearly mintage as plated, and the remaining in a solid core coin.
I had fun looking for early 2000's variations of 'no-P', 'P' (2000-2002), then it went to old vs new bust (2003), then there was in the case of 2006 pennies 'no-P', 'P', RCM-zinc, and RCM-steel... you may need to find a charelton book to know what was, or what wasn't minted by denomination and year - but you can buy a used book, as the values don't rapidly change on recent coinage, and it's nice to have a book to at least help you with major variations.

Welcome here, and welcome to the hobby!