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Since you are using a block theme, this is quite simple. Go to Appeareance > Editor > Templates > Manage all templates. There you will find a list of all templates. For those that are customised, you will find 3 dots on the right with “Clear Customizations”. This would reset the template completely.
The customisations you have made will of course be lost. Content will not. If you are unsure, create a backup beforehand.
Nice if I could help. You are welcome to set the topic to solved.