You are always either long or short

Even if you don't have any interaction, any account in some financial broker or whatever, you are holding a position, think about it.

If all you got is $1000 in your wallet, then you are longing the US Dollar while shorting everything else.

If, let's say, your equity is $1000, but you hold $500 cash and $500 worth of gold, then you are longing these both instruments, the US Dollar currency and gold.

A day will pass, then a week, a month, and your portfolio will either go up or go down.

If you got $1000 US Dollar in cash, but think that maybe it can lose it's value against the Euro by next week, then you can protect yourself by moving your equity from US Dollar to Euro, basically longing EUR and shorting USD, surfing the market.

We could go a little bit further... Imagine that you are an Iron miner/distributor, and you are signing a private contract to deliver $10M worth of Iron to company X by next month. You are technically shorting Iron and longing USD. You start from the assumption that the price of Iron will not increase dramatically until delivery date, otherwise you would already sell it with a corrected price.

If on delivery date you don't have the Iron to sell, because some thing happened in the pipeline like machines were blown up whatever, you will need to buy it elsewhere to fulfill your client request.

We got the commodities markets for this but you got the idea.

How would you weight your portfolio then?

Well, this is a deep topic, and I am by no means an expert.

I guess an portfolio should be calculated and recalculated every freaking nanosecond, or as frequently as possible, given that people expectations of the future are constantly changing.

But from an universe of financial instruments, be it metals, commodities, crypto, currencies, stocks, etfs, and even trading strategies (if you treat them like financial assets, why not? fund of funds..), you would use Modern Portfolio Theory (MPT) and attempt to find the Efficient Frontier.

But at the end, it's all about finding the Expected Value of a trade, a bet, a position...