Therapists are licensed by a state board. Legally this means that they must see clients who are physically located in that state. This can either be by:
Meeting in-person (within the geographic bounds of the state)
Meeting virtually with video teletherapy, where the client is physically in the geographic bounds of the state
Some therapists hold licenses in more than one state, but regardless of how many licenses they have, where the client is physically located is where the therapist must have a state license.
In special cases, such as with the COVID-19 pandemic, states may relax their jurisdiction requirements and allow therapists to practice in their state if they hold a license in another state. These exceptions are rare and usually for limited lengths of time.
Teletherapy:
While teletherapy is now booming (thanks to COVID-19), even prior to this there were several companies who were, and still are, only online (such as TalkSpace, BetterHelp, or Faithful Counseling). With these services, you are matched with a therapist who is licensed in your state of residence.
Teletherapy is still bound by the laws of each state and the requirements of licensing jurisdictions. A therapist must hold a license in the state where their client is physically located during teletherapy.