What units is volume measured in?

I have been trying to find an answer to this question (in 2 parts) for quite some time now.

The most reasonable, understandable definition of volume that I can find is that it is the number of units of a currency pair traded for a given candle.

So, for example, if we are looking at the 1D EUR/USD chart, and the volume for a candle reads 5,000, that means “volume(1D) = 5,000 EUR/USD” (or could it mean “volume(1D) = 5,000 USD”?).

The first question I have: is the above an accurate definition?

The second, related, question I have is this:

If someone buys one hundred thousand (100,000) units (1 lot) of EUR/USD, will the Volume of the EUR/USD chart increase by one hundred thousand (100,000) or ten (10.0000), because in the latter case 1 USD pip = 0.0001?

If we look at yesterday’s 1D candle on TradingView for EUR/USD on OANDA, the volume was 150,741.56057. If we consider the average daily volume for all Forex is 6.6 trillion, I hope you can see why this second question also gets raised: because even if the volume on that particular chart is just for that particular broker, 150,741 and some change is a whole lot less than 6,600,000,000,000. And even if we factor in 1 pip = 0.0001, that’s still 660,000,000.0000 (660 million).

The only way I can see to rationalize this is to say that the average daily volume for all Forex is 6.6 trillion USD but I am only considering the 1D volume for EUR/USD in my comparison, and those 2 units are incompatible, which is the reason why they are wildly different. That’s just a guess but it seems reasonable enough.

Actually it looks this question is in three parts:

1. Is “the number of units of a currency pair traded for a given candle” an accurate definition of volume?
2. If so, which units?