In short: Inside one OneCode discount, each item only gets its single best product discount, and only the biggest order discount is used. Across different discounts, Shopify only combines them when both are set to combine, and gives each item one product discount.
Inside one OneCode discount
Product discounts: if two product discount types target the same item, the item gets the one that saves the most. This is how tiers work: 10% at 2 items and 15% at 3 items gives 15%, not 25%.
Order discounts: only the order discount type that saves the most is used.
Different kinds always stack: the winning product discount, order discount and shipping discount in the same code all apply together.
If you want two discounts to add up on the same item, combine them into one discount type with the total percentage, or make one of them an order discount.
Between OneCode and your other discounts
Both discounts must allow combining with each other's kind. Check Combinations on both.
Even when product discounts combine, Shopify gives each item only one product discount: the one that saves the most.
Shopify only allows one shipping discount per order.
On Shopify Plus, product discounts can stack on the same item using tags. See How OneCode discounts combine with your other discounts.
Good to know
Customers can only enter a second code at checkout if the first one allows combining.
A OneCode code with product, order and shipping discount types counts as one code for the customer.