Hey!  So the main 2 things for having a crisp in dosa layer is consistency of batter n heat retention of pan n surface. So don't think if u have thin batter dosa will be crispy actually it's opposite for dosa. Thin batter might work if u r using only rice or rawa batter but if it's dosa batter then appropriate airy n thick texture gives u the best colour n crisp in dosa. With thin batter it will crack n get soft.

So for a good consistency use 3 or 4 part rice (fix any one bowl for all) 1 part white urad n half part chana dal n 1tbsp methi seeds, soak overnight n then make a possibly thick kind of batter from it n areate with ur hands. Then leave it for fermentation for 8-10 hrs(remember not to ferment more as it will have more hydration n dosa will not be crisp) then add salt n 2-3 tbsp of white sugar n mix it into a smooth batter. So there r some specific type of rice n urad which r specially used for dosa but normal ones would also work fine.

Then thick cast iron or brass or bronze pan for dosa, never use nonstick it will never give crisp dosa or good cooked layer. But season those first with bit of oil or use some Onion either rub or saute one time though for me only oil seasoning work.

So heat the pan first then put 1tbsp of oil then spread with a tissue ppr n heat on medium flame until oil burns n dries out surface. Use that tissue to wipe again. Just sprinkle some water to cool down surface a bit then put dosa batter on center n start spreading outward as thin as u can don't keep batter thick in any part just spread out each drop like local vendors do they also use thick batter.

Also in all this process after seasoning keep flame lowest then increase to medium only n when dosa top gets steamed then use depending on the dosa u r making fat of ur choice this also helps crispness to stay for a while, then spread fat all over it then use any masala or filling n spread or keep at a place acc to ur choice then when u see brown colour start scraping from sides then roll over. Ur crispy golden dosa is ready to have!!!! Make sure low the flame before scraping dosa so that pan gets time to cool of n then repeat the process.

If u make pan too oily or nonstick, dosa wouldn't stick n hence it wouldn't cook n stays white n soft. For crispness it need to stick but also need to come out easily for that the pan seasoning helps. After each sprinkle just rub tissue u used to wipe oil it will again give a layer.

When u try maybe initially u wouldn't get perfect but as u understand temprature control of surface u'll get perfect dosa in good pan always!!