PSA, do NOT eat or use the outer part of an Ethrog/Etrog/Esrog for food, that’s where all the pesticides is and there’s a lot of it.

Ethrogs are doused in large quantities of pesticide because of the need for them to be even less free of blemishes then most other products as well as their limited season puts even more pressure on having as much of the crop be fit for sale as possible.

https://www.reddit.com/r/Judaism/comments/75agaa/warning_against_using_etrog_for_jam_or_vodka/

Having said that, all the pesticide traces will be concentrated in the outer part of the fruit so if you only use the inside you should be fine.

https://en.toraland.org.il/beit-midrash/articles/around-the-jewish-year/sukkot/holy-etrog-jam-and-pesticide-traces/

Also not all Ethrogs are grown like that, some are safe to use the whole thing: https://www.esrogfarm.com/pesticides-and-esrogim/

Edit: https://www.myjewishlearning.com/the-nosher/is-it-safe-to-eat-your-etrog/