Marks are used as source identifiers for particular classes of goods and services.
For example, the mark Xerox identifies that a particular copier came from the source Xerox corporation. Marks are adjectives: Xerox copier = a copier from Xerox. Not a Xerox (used as a noun). If a mark holder uses its mark as a noun,or allows it to be used as a noun by others (like Bayer did for its Aspirin pain reliever), it loses the right to exclusive use of that mark as a source identifier.
But Xerox is only a source identifier for a limited class of goods and services. So, for example, I could market a Xerox coffee without infringing Xerox's mark. In other words there can be many, many Xerox marks (until such time - if ever - that the mark becomes so famous that allowing inferior products to be marked under the mark would dilute its value.) (In fact, Xerox is that famous - so I could actually be sued if I marked Xerox coffee. But very few marks reach that status.
Meta's mark was originally filed by Microsoft and assigned to Facebook, then Meta after use in a way that does not in any way resemble how Twitter is using its X. The mark is an image mark - with a particular style. Here is the description: The color(s) white and blue is/are claimed as a feature of the mark. The mark consists of a stylized letter "X". The left side of the "X" is white and the right side of the "X" is blue. The shaded square carrier represents background only and is not a part of the mark.
Here is a visual: https://tsdrsec.uspto.gov/ts/cd/casedoc/sn87980831/SPE20190508154150/1/webcontent?scale=1
I haven't done enough trademark use recently to opine on the use of very different letters as source identifiers for very simliar goods and services. But if Meta wants to keep Twitter away from the X, it will need to actually use it - since registration presumptively establishes that Meta owns the (stylized) mark - but Twitter can seek to invalidate the registration (one means would be non-use).
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