What is SPX500USD?

This is a Contract for Difference (CFD).

According to the TradingView page, it is an instrument from OANDA, a forex and CFD broker. Specifically, it is a Contract for Difference (CFD) traded on OANDA. See: US SPX 500.

How the price is set (reference):

3.3 Our price for index CFDs

3.4 Our pricing system receives index futures pricing feeds from relevant futures exchanges. We review the "top of book" prices (the best actionable prices) (the highest buy and lowest sell) from those feeds to calculate our Midpoint Price, as described below. The futures prices of indices is affected by dividend payments and corporate actions on the underlying index as well as interest rate changes.

3.5 We then use an automated adjustment schedule for each index, which adjusts the price of the relevant index futures at the date of the transaction based on changes set out in that schedule.

3.6 In addition, we apply a daily cash offset by way of a credit or debit to your account which mirrors the adjustments made in the dividend schedule. Such adjustment is made on the dates specified in the dividend schedule.

3.7 Below is an example of how a futures price might be calculated on an index CFD, with stepped dividend adjustments between the near and the far futures contract, along with an example Dividend Schedule. The chart assumes the futures price remains flat rather than fluctuates as it would in practice: