The ability to stack discounts on a single order has been a major Shopify limitation for a long time as it was only possible to apply one discount code to the cart.
Store owners on the Plus plan have been able to use the Script Editor app to write Line Item Scripts that can potentially apply multiple discounts, however, this functionality has only been available to merchants on the enterprise plan.
This all changed with the recent product updates announced at the summer Editions event in June 2022.
The newly released Discount combinations allow Shopify buyers to applying multiple discount codes to the cart at Checkout:

This new feature is availble to all merchants, regardless of the store plan. Product discounts can be combined with other product discounts, for example 10% off Jeans (10OFFJEANS) and 20% off T-Shirts (20OFFTSHIRTS):

It is also possible to combine different types of discounts, for example a 10% off Jeans discount with a Free shipping discount.
Discount combinations are one of a long list of improvements to Discounts that were announced at Editions, so make sure you check out all of the new features and start experimenting with different strategies.