In 2020.1 PsychoPy created an html folder for index.html and the javascript files. It also copied resources to html/resources and loaded everything in that folder into memory at the start of the experiment. If you needed a resource that was only mentioned in code you could just copy it to html/resources to tell PsychoPy to load it.
This was inefficient because it meant that most experiments had two copies of its image, sound and video files which could nearly double the disk space used.
From 2020.2 the default became creating index.html and the JS files in the experiment’s root folder, alongside the psyexp file. If you needed a resource that was only mentioned in code you now have to add it via Experiment Settings / Online to tell PsychoPy to load it.
From 2021.1 PsychoPy can get confused by experiments still using the html folder and also needed to add resources via Experiment Settings / Online. If it works, don’t worry about it, but if it doesn’t then I would recommend recreating the experiment in a new local folder and syncing it to a new online experiment, having first ensured that the output path in Experiment Settings / Online is blank.