The lower thread (bobbin thread) is wound onto the lower thread spool before sewing. It is inserted into the bobbin case under the base plate of the sewing machine and forms the lower part of the seam with a predefined lower thread tension.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 211