Are there Goddesses in Buddhism? If yes, who are they? If they exist, then how are they connected to the God(s)/Lord Buddha?
I'll answer in two parts-- what sort of looks like a god in Buddhism and which of those are female.

Early Buddhism didn't see gods as important to solving their fundamental problem of ending suffering. Gods were seen as a possible destination for our reincarnation. Gods didn't provide any help to us humans, they were too busy enjoying themselves (devas) or jealously fighting each other (asuras). In Mahayana Buddhism, the devas are not the target of devotionalism, respect and appeal for help, so I'll skip over them. (Except maybe to the extent that they were recast as Bodhisattvas &amp; Buddhas)

Later Buddhism developed the Bodhisattva Path, which was based on the idea that one should strive to become a Buddha. The Bodhisattvas were models of behavior, but rapidly also became devotional targets who would provide various this-worldly benefits, such as defense against snakes.

Also by this time, the Buddha and enlightened people in general were supposed to have extreme long life, the ability to know past lives, to do "astral projection", and so on. As sutras evolved, Buddhas gained more powers until they were indistinguishable from a conventional god from a more familiar religion like devotional Hinduism, Christianity, etc.

There was a lot of controversy about if a woman could reach enlightenment (arahathood) or become a Buddha (full Buddhahood). I think it's not controversial to say that from the time of the historical Buddha the relevant societies were deeply sexist and reluctant to admit that woman could become Buddhas.

Avalokiteśvara was Bodhisattva who once the story arrived in China became a woman (Kwan Yin). Kwan Yin is both a devotional target and a model.

Tara is an important Yidam in the Tantric system. People appeal to Tara both for this-worldly aid and use visualization techniques to imagine themselves as a Tara-- i.e. an enlightened being.

Jan Natier has a good discussion about women in Buddhism, in this paper, although it is is somewhat specific to the Lotus Sutra by the end: https://www.academia.edu/8164216/Gender_and_Hierarchy_in_the_Lotus_S%C5%ABtra