The Monster Manual tells us that:
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.
What are these actions? Are they explicitly listed in the rules?
The "Actions" section from the introduction to the Monster Manual says (p. 10; 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.
(The corresponding part of the basic rules is missing this clause.)
So we need to find where actions such as Dash and Hide are described in the Player’s Handbook. They are described in Chapter 9: Combat, under "Actions in Combat" (p. 192-193), or here in the basic rules:
When you take your action on your turn, you can take one of the actions presented here, [...]
Here're all the actions presented:
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 actual list might differ between different DMs, depending on their playstyle. The vast majority also explicitly includes Grapple and Shove, which are described as "the most common contests" (but technically they are still a part of the Attack action).
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" and keeps options open for DMs and players alike.
If you want to get a specific exhaustive list of all the actions available to all creatures, this is probably a dead-end because creatures differ.
This page lists several actions that may be taken in combat. It's not exhaustive, but it does provide a good guideline of what to expect another creature to be able to do. There is even a helpful blurb that explains you can improvise actions; this would apply to NPCs as well.
The gist seems to be... if it doesn't require a class feature, racial feature, or magic item/feat, an npc can perform that action.
This isn't to say this is all of the actions available to players or NPCs, in fact there are some (like stabilizing someone with a medicine check or using certain objects) that have their rules written else where, but it's a good place to start.
