This is mostly about “Actions in Combat” section from the PHB
This is the whole paragraph (emphasis mine):
When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player’s Handbook.
So we need to look up Dash or Hide actions in the Player’s Handbook. They are described in the Chapter 9: Combat, “Actions in Combat”:
When you take your action on your turn, you can take one of the actions presented here
Here’re all the actions presented:
- Cast a spell
- Use an Object
- Improvised Action
This is not an exhaustive list of what creatures can do. For instance, many living things can also eat, sleep, talk, et cetera, but all these things are not relevant in combat.
The list might differ from one DM to another. The vast majority also explicitly includes Grapple and Shove, which are described as “the most common contests”.
The list above is primarily about actions in combat. They are described in the respective chapter, and this is important, because context matters:
In any piece of writing, context matters. If a rule has multiple sentences, they’re meant to be read together.
It also has an “Improvised Action” option, which is basically “any other action the creature can plausibly do”.
If you want to get a full list of all the actions available to all creatures, this is probably a dead end, because creatures differ.