short answer: "\data\side_r.csv" and "\data\side_l.csv"
https://assettocorsamods.net/threads/how-can-i-make-ai-path.234/
or my version of it:
Recording ai-line tutorial
(!Disable NewAiBehaviour or complete CustomShadersPatch!)
(AI app only enabled with "developer apps" ON)
Record "ai\fast-lane.ai":
-Run game in practice or hotlap mode (maybe holding shift if you have side_l/r.csv already)
-Go just before the starting line and stop
-press record in AI app and drive
-first lap is mandatory (but not what we want) - every lap after counts and can be used (or saved)
-if you like a lap you have driven, hit "Esc" and "Exit" and just exit assetto...it saves in background, it will take a while to actually exit, longer for long tracks
-now you should have a "ai\fast_lane.ai.candidate"
rename it to "ai\fast_lane.ai"
-copy it to "data\ideal_line.ai" (i think its for ai-fuel-calcs)
-start in "Race" mode now and control if ai line and track borders are good with ai app on, you can use Ctrl+C to switch on the AI driver
Record "ai\pit_lane.ai":
Run game in practice or hotlap mode, go just before where the pitline would split off the existing ai line (ai app on). Be exact as possible, if not ai will wiggle unforseen in game, when going off the normal line to the pit line.
Then hit "record pit" or what its called, then drive through the pits, no need to stop or anything, just drive. Then stop when you reached the normal track ai-line, again as exact as possible, and exit the game.
Rename "ai\pitlane.ai.candidate" to "ai\pitlane.ai"
Now about track-borders (red with AI app on)
-After recording track borders might not be like you want, they could have spikes or go wide to the barriers and such
-1- You can correct that with <
https://www.racedepartment.com/downloads/ai-line-helper.16016/>
-or-
-2- This blender addon lets you import ai lines (including sides) and export side....csv:
https://assettocorsamods.net/threads/blender-addon-for-import-export-csv-and-ai-files.2266/
-A tip from Liski for recording:
in "surfaces.ini" set "IS_VALID_TRACK=0"
for all other surfaces than "ROAD" and "KERB" maybe
and you should get nice and crisp track borders
WITHOUT recording "data\side_l/r.csv"
if its layed out correctly in the 3d model
-if you have "\data\side_r/l.csv" files, simply hold SHIFT while the game is loading, AC will read them and integrate them into the current AI line, with AI app on there are now two ai-borders, red and yellow for the newly imported borders
