A high-level process for authenticating and trusting a Member State Trusted List (TL) is defined in Annex A of ETSI TS 119 612 v2.1.1 (this standard is prescribed in eIDAS via CID 2015-1505).
The source of trust of the TLs is the EU List of the Trusted Lists (LOTL). And the source of trust for the LOTL is the Official Journal of the European Union (OJEU). Initially, signing certificates of the LOTL are defined in https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=uriserv:OJ.C_.2016.233.01.0001.01.ENG.
Further signing certificates of the LOTL are defined in the LOTL itself, in LOTL publications named "pivot LOTLs". All pivot LOTLs are present in the SchemeInformationURI of the current LOTL. Currently, these pivot LOTLs are:
In these pivot LOTLs, the signing certificates (i.e. ServiceDigitalIdentity) of the LOTL are defined in the OtherTSLPointer for which the SchemeTerritory is EU (i.e. the pointer to the LOTL).
Therefore, when trusting the signing certificates first defined in the OJEU, you will be able to trust signature on pivot LOTL 172. Then, when trusting signing certificates defined in pivot LOTL 172, you will be able to trust signature on pivot LOTL 191. Same for signing certificates defined in LOTL 191 to trust signature on pivot LOTL 226. Finally, with signing certificates defined in LOTL 226, you will trust the signature on the current LOTL (i.e. https://ec.europa.eu/information_society/policy/esignature/trusted-list/tl-mp.xml).
This "pivot LOTL" mechanism is introduced in the above OJEU publication (where compiled list = LOTL):
Any future change in the ‘secure pointer to the compiled list’ shall
likewise be reflected in the publication of a new instance of the
‘compiled list’ which will include a new URL or a modified set of
digital certificates for relying parties to use when authenticating
the ‘compiled list’