If you want to play Texas Hold’em against other poker players online, you need to go to a poker room. This is true for ring games (cash games) as well as for tournament poker. Many big gambling sites with an online casino will also feature a poker room.
If you want to play a Texas Hold’em-based game of poker against the house, this is also possible. When you play against the house instead of playing against other players, it is called casino poker. Casino poker varies from traditional poker in several important aspects. Casino poker based on Texas Hold’em poker is available both in normal online casinos (where the outcome is determined by an RNG) and in live dealer casinos online (where you play against a real dealer who works in front of web cameras).
One example of a live dealer casino studio online that offers casino poker based on Texas Hold’em poker is the one run by Evolution Gaming. You can access this live dealer casino from several different gambling sites, including Unibet (go to Casino Grande), BetVictor and Betsson. The game is called Live Casino Hold’em.