Right - if the POTUS is unable to serve the first replacement is the vice president.United States presidential line of succession
Current order of succession
No. Office Incumbent
1 Vice President Kamala Harris
2 Speaker of the House of Representatives Nancy Pelosi
3 President pro tempore of the Senate Patrick Leahy
4 Secretary of State Antony Blinken
If the president AND vice president are unable to serve the speaker of the house becomes POTUS.
If the president AND vice president AND speaker of the house are unable to serve the president pro tempore of the senate becomes POTUS.
In each case, the vice presidential position is filled by an appointment from the new POTUS with congressional approval.