I don't know what browser or OS you're using, or what website you visited, but:
GTS Root R1 has self-signed certificates, but it also has a certificate issued by GlobalSign's GlobalSign Root CA, an older and widely-trusted CA. (The two crt.sh links show this in the Certificates and Parent and Child CAs sections.)
If I access https://www.google.com/ right now, the certificate chain I see is:
www.google.com issued by GTS CA 1C3
- GTS CA 1C3 issued by GTS Root R1
- GTS Root R1 issued by GlobalSign Root CA
If GTS Root R1 is in your trust store, you can short-circuit validation at that point and trust the certificate. If it's not, you can validate the GTS Root R1 certificate if you trust GlobalSign Root CA.
If Google didn't want to rely on GlobalSign Root CA, they wouldn't serve the last certificate in the chain at all.
The practice of bootstrapping new CAs using older CAs is quite common, and is referred to as cross-signing.
Sometimes a new CA buys an entire root certificate from another CA that has several roots and can afford to part with one for the right price -- in fact, Google acquired GlobalSign Root CA - R2 from GlobalSign, but it expired in 2021 and they had to switch to the current solution.
Google blogged about the change at the time.
(P.S. The industry has gone through so many mergers, acquisitions and rebrandings since the 1990s that the name of a CA certificate and the name of the company that owns it can be completely different, even if the certificate itself has never directly changed hands.)