The UA doesn't say - but I'd rule that modified/replaced features (that your DM allows) act like errata to the original
According to the introduction to UA: Class Feature Variants:
Here you’ll find features that replace or enhance the normal features
of your character’s class. The class feature variants each specify
which feature they replace or enhance, as summarized in the Class
Feature Variants table (see page 2). If a feature is replaced, you gain
no benefit from it and don’t qualify for anything in the game that
requires it. If a feature is enhanced, you continue to enjoy its
benefits but now with new capabilities.
The DM decides which of these options are available to the characters
in a campaign. A DM is free to prohibit these variants, allow all of
them, or make a subset of them available to you.
Let's assume that all of the options from the UA are allowed for simplicity. Then if we look at any given class feature enhancement, such as Metamagic Options for the sorcerer:
3rd-level sorcerer feature (enhances Metamagic)
When you choose Metamagic options, you have access to the following
How I'd interpret this is that, effectively, the list of Metamagic options goes from the list in the PHB to [the list in the PHB + all of these extra options]. Nothing in UA: Class Feature Variants says the abilities only get enhanced/replaced once you reach those class levels - it's just that the enhancements/replacements usually don't have much effect unless you have taken levels in those classes, most of the time.
Then if we look at the Metamagic Adept feat in UA: Feats (2020):
Prerequisite: Spellcasting or Pact Magic feature
You’ve learned how to exert your will on your spells to alter how they
function. You gain the following benefits:
- You learn two Metamagic options of your choice from the sorcerer
class. You can use only one Metamagic option on a spell when you cast
it, unless the option says otherwise. Whenever you gain a level, you
can replace one of your Metamagic options with another one from the
Basically, it lets you learn 2 options from the sorcerer's Metamagic feature. It doesn't specify a list of options you can choose from, or say you're limited to the ones in the PHB; it just has you reference the sorcerer's list of Metamagic options.
Now, although the interaction of content from one UA with content from another UA isn't necessarily well-defined, then whatever things you can choose from for the sorcerer's Metamagic feature (whether from UA: Class Feature Variants or another source), you can also choose those Metamagic options for the Metamagic Adept feat. No additional restrictions exist in the content preventing you from choosing those options if they've been allowed as options for a sorcerer's Metamagic feature.
The same applies to the Magic Initiate example in picking from a druid's expanded spell list. The Druid Spells feature in UA:CFV says:
1st-level druid feature (enhances Spellcasting)
The following spells expand the druid spell list.
Cantrips (0 Level)
Here, it expands the druid's spell list.
The Magic Initiate feat says (PHB, p. 168):
Choose a class: bard, cleric, druid, sorcerer, warlock, or wizard. You
learn two cantrips of your choice from that class's spell list.
This lets you pick any of the listed classes, and learn 2 cantrips from that spell list (as well as a 1st-level spell that you can cast once per long rest). If the Druid Spells enhancement from UA:CFV is being used, then acid splash becomes a valid cantrip option for those who take the Magic Initiate feat and pick the druid spell list for it. (Likewise, ceremony and protection from evil and good become additional options you can pick from for the 1st-level spell that you can cast with the feat once per long rest.)
In essence, any class feature variants that enhance existing class features, if allowed by the DM, make it so that the feature then reads as if it always included the enhancement. Any class feature variants that replace existing class features make it as if the replacement is what the class gained at that level to begin with. The class feature variants, as presented in UA:CFV, don't have class level prerequisites that must be met before you're allowed to act as if they exist; those variants that your DM allows are basically treated like errata to the original feature they enhance or replace.