Well, with 20/20 hindsight, Texas didn't have standing, according to SCOTUS' decision:
The State of Texas's motion for leave to file a bill of complaint is denied for lack of standing under Article III of the Constitution. Texas has not demonstrated a judicially cognizable interest in the manner in which another State conducts its elections. All other pending motions are dismissed as moot.
Of some note, Alito and Thomas dissented on the[ir] principle that all original jurisdiction cases need to be heard [in their view]:
Statement of Justice Alito, with whom Justice Thomas joins:
In my view, we do not have discretion to deny the filing of a
bill of complaint in a case that falls within our original
jurisdiction. See Arizona v. California, 589 U. S. ___
(Feb. 24, 2020) (Thomas, J., dissenting). I would therefore
grant the motion to file the bill of complaint but would not
grant other relief, and I express no view on any other issue.
(From some stats I've seen, some 50% of original jurisdiction claims are rejected in this first phase though; by 2018: "In the three decades since 1961, only 102 motions asking leave to commence original jurisdiction actions have been filed by or against a state [...] Of those motions in state-party cases, the Supreme Court has denied 50, nearly half".)
To add this seemingly important bit to Ryan's answer, those 4 states merely changing their laws is apparently not enough for an actual injury. The claim also has as its final bullet point in its lead:
The appearance of voting irregularities in the Defendant States that would be consistent with the unconstitutional relaxation of ballot-integrity protections in those States’ election laws.
Which is almost certainly why the suit contains all those statistical attempts to prove that absent those laws-changes/"irregularities", Trump+Pence would have won.
So, yeah, the suit argues that (1) state election law/regulations changes (essentially that made mail-in ballots easier in 2020) were unconstitutional and (2) Trump+Pence would have won in the absence of those changes, i.e. had laws been enforced the same as in 2016.
A bunch of AGs (and governors) from other states have now filed briefs in opposition to that. To quote from Montana's counterarguments:
Texas’s claims under the Electors Clause hinge on its
theory that voting procedures in Pennsylvania,
Wisconsin, Michigan, and Georgia conflict with the
enactments of those states’ respective legislatures. But
state election laws vary considerably. Hence, even
accepting Texas’s premise that a state-law objection to
a voting procedure could yield a federal constitutional
violation, the Electors Clause analysis necessarily
differs from state to state. Likewise, the susceptibility
of state voting procedures to fraud, as well as states’
historical experiences with fraud, may differ from state
to state. It is impossible for such allegations to be
properly adjudicated in a single Original Action
involving four States, with no factual record or legal
conclusions by any lower court, on a highly expedited
time frame.
When litigants did bring state-by-state
challenges before the election, those challenges proved
unsuccessful. Montana’s experience is a prime example. [...]
Of course, the Supreme Court has to decide on these conflicting viewpoints.
N.B. There's already a fairly lengthy Wikipedia page on the case. I won't try summarize the various commentaries here, but interestingly perhaps, Alan Dershowitz
(who had defended Trump during impeachment) opined that the Texas suit was "creative but unlikely to win".
