ZeroSSL (https://zerossl.com) is a product of a for-profit company (HID Global). They are not a certificate authority per se, but a reseller for Sectigo certificates (that is, all issued certificates come from Sectigo with a white-labeled intermediate).
ZeroSSL offers free certificates via ACME like Let's Encrypt. However, they also have non-free offerings on their website, so not everything on ZeroSSL is free (unlike Let's Encrypt).
ZeroSSL finances and owns several ACME clients (most of which were originally developed for Let's Encrypt). This includes acme.sh, win-acme, and several more. As such these ACME clients may default to ZeroSSL instead of Let's Encrypt (for promotional reasons, probably).
The documentation on your ACME client should have instructions how to switch CAs, should you wish to change to Let's Encrypt.