An ASSET can serve different purposes, which will define how you approach their production.
There are different categories of ASSETs including Blocks, Entities, Equipment, and Art.
Knowing the function you want your ASSETs to perform in your Experience will dramatically change how their development is approached, so make sure you are clear on what ASSETs you will create, and what they will be used for.
For example, almost any ASSET in the Game Maker can have functionality attached to it through Behaviours and Components, but an Entity that has been rigged for movement will be far more effective than a static ASSET.
These are environmental ASSETs that change the look of the world. Some basic blocks are already available (ie: water, mud, sand). But artists can create unique blocks for more unique worlds (ie: purple water, cute sparkly lava, mud blocks of a lighter soil type).
Blocks are not 'functional' ASSETs - they can't have Behaviours or Components applied to them. You can, however, set blocks to deal damage when an Avatar collides with them.
These ASSETs are often referred to as non-playable characters (NPCs) such as people and animals. In other words, entities that inhabit the world the player will be exploring. For example, a farmer, a chicken, a dinosaur, a treasure hunter and so on.
They could be neutral characters used to start quests, allies ready to offer guidance, or enemies waiting to defend their territory.
These are NFTs that can be placed in the player's inventory. They can be equipped in order to help the player complete game objectives. For example, a sword, a shield a helmet, a pair of gloves and so on.
These are NFTs that serve mainly as visual decorative items to add some wow factor into games and Experiences. For example, framed artwork, statues, landmarks and so on.