Based on Flux documentation (most relevant entry below), here’s what Flux says about creating schematic symbols — quoted from the docs verbatim:

"Flux works slightly differently than other PCB design tools you might be used to. In Flux, parts have two different views: the schematic and the symbol. The schematic view only contains the terminals, while symbols are only visible when a part is placed into a project.

To see a part's symbol at any point in the process, you need to:

...

If the default parametric symbol does not work for your use case, you can create a custom symbol. We suggest only advanced users try this process as it requires external tools.
Refer to the Custom Pad Shapes tutorial to learn how to create a custom symbol shape. Once you have the asset created and added to Flux, proceed to matching pin locations.

Matching Pin Locations
By default, all terminals will be located at the center of the symbol. To position the terminals to the desired location, there are a few more steps. We've covered this process both in video and written format.

Publish the part
Create a New Blank Project and drag the part you're importing.
You'll notice that both terminals are at the center of the symbol. Now go back to the imported part.
You'll need to do this process for every terminal in your part

Select the terminal and find the "Properties" menu in the right-side panel.
In the "Symbol Pin Position" field, type the desired x and y coordinates for the terminal to sit on the symbol.
Publish the part and go back to the new project. You'll see a "Update available for your parts" legend in the bottom left. Click on "Review" and accept the changes.
You'll notice that the terminals have moved. You might need to repeat this process a few times to nail the perfect position."

Source: Flux — Working with Schematic Symbols

Additional Flux knowledge-base guidance to follow:

If you want, I can:

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