Hi there!
Thanks for reaching out to us.
- If you are talking about the storefront then you will find it within the SP Page Builder Pro. Here is a screenshot for your convenience:
https://prnt.sc/H2znh5gEmsnZ
If you open the storefront with SPPB editor then you will have thumbnail addon there and from the advanced tab you can adjust custom width of it:
https://prnt.sc/3GwX7hSpaX94
Here is the single view with frontend: https://prnt.sc/acVB2AUzvJ3F
- Single product view, you can add video addon from SPPB and the video addon will allow you to embedd you tube videos there:
https://prnt.sc/ktbVGes0Wha4
- Yes, you can add custom fields for single product view page. Just add new row and your desired contents there.
https://prnt.sc/9TPo_k0fDxSW
There is no default way to change the checkout page appearance without coding.
Best Regards