If you’re interested in apply for Google Summer of Code, you may be interested in see an example of a proposal. I’ve embedded my proposal for GSoC 2017 below.
GSoC Proposal to work on the Plone Theme Editor.
Please note that the Plone Foundation is not participating in GSoC 2021. For the list of organisations for this year GSoC, see https://summerofcode.withgoogle.com/organizations/