Sequential ultrasonic spot welding is an interesting joining method for overlapping thermoplastic composite structures. In the framework of the EU Clean Aviation Multi-functional Fuselage Demonstrator (MFFD) and the lower shell SmarT multifunctional and INteGrated TP fuselage (STUNNING) projects, SAM XL and TU Delft Aerospace Engineering collaboratively developed and demonstrated a robot-based sequential ultrasonic spot welding process for the sub-assembly of structural frames and clips in a fuselage section demonstrator. This full-scale thermoplastic composite fuselage section demonstrator, which was recently awarded the 2025 JEC Innovation award, measures 8.0 m in length and 4.0 m in diameter. Our robot-based sequential ultrasonic spot welding technology played an important role in ensuring the joining of structural clips and frames in the stiffened fuselage skin of the demonstrator, through the use of more than 1600 spot welded joints with an average welding time of approximately 10 s per spot, thereby significantly reducing cycle times as compared to traditional joining methods such as fastening or riveting. This paper provides a comprehensive overview of the technology development process and highlights the results achieved during the sub-assembly of the demonstrator, as well as the challenges encountered.