Depends on the screen size.
The 27-inch has an access door to upgrade the RAM memory, so is simple and straightforward.
The 21.5-inch is not easily upgradeable. The only possibility is hard drive/SSD. The RAM is soldered to the logic board.
Hard drives in either size are a challenge to upgrade, because of the need to literally slice open the enclosure.
Solid-state storage is a challenge to purchase upgrades.
Neither size uses a standard SSD, but uses slot-type PCIe proprietary storage.
Right now, there are only a couple of companies that have upgrade SSDs, or you can find Apple-produced drives on eBay, etc.