This is a Rules-as-Intended (RAI) item from the Sage Advice Compendium, which nobody in this thread has quoted yet so I will provide it to you here.
When you cast a spell like conjure woodland beings , does the spellcaster or the DM choose the creatures that are conjured?
A number of spells in the game let you summon creatures. Conjure animals, conjure celestial, conjure minor elementals, and conjure woodland beings are just a few examples.
Some spells of this sort specify that the spellcaster chooses the creature conjured. For example, find familiar gives the caster a list of animals to choose from.
Other spells of this sort let the spellcaster choose from among several broad options. For example, conjure minor elementals offers four options. Here are the first two:
- One elemental of challenge rating 2 or lower
- Two elementals of challenge rating 1 or lower
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option. For example, if you pick the second option, the DM chooses the two elementals that have a challenge rating of 1 or lower.
A spellcaster can certainly express a preference for what creatures shows up, but it's up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
Being RAI means it is not explicitly called out in the rules and thus whether or not someone agrees with this ruling is up in the air. This is a large part of the problem with 5e's use of natural language in spell descriptions, wherein Crawford acts as if he used formal language. For instance, a Druid's Wild Shape says "Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before" but doesn't explicitly say it's the player's choice. We all know it's the player's choice, but the same sort of logic being used to argue the point in the excerpt above could easily lead you to the conclusion that it isn't. Natural language is, and always will be, open to interpretation. If they wanted to use natural language, they should have been prepared to explicitly define every intent.