Is there a chess simulator where I can practice certain lines?

I think ChessTempo might have something for practising openings where you can put in the moves for black and white and play through one side while the computer does the other. Also you can write your own books on Chessable and use its Spaced Repetition to learn the variations. I haven't tried custom books on Chessable but you get a few free slots with a basic account and I think these can be used for custom books.

I think you can create a FEN for the starting posizion of your choice and then play against whicever computer you like!
you can play against the max level computer here from any position, try the 'learn' menu, analysis board. But you can't play like one of the bots from a position.
You can do the same on many a you vs your computer program, set it up and play against it, and there you usually CAN set the difficulty.

Yeah, the chess.com analysis board is great for exploring these kinds of things. Just be aware that it's only available on the website, not in the app (for some weird reason.)

I think you can create a FEN for the starting posizion of your choice and then play against whicever computer you like!
+1

You can practice by playing against a bot starting from a given position, if it's what you want. I know 2 ways:
1. You go to analyse and choose setup or openings. Once you reached the initial position you click on the icon 'vs, computer'. (Bottom right on the screen)
2. You go to openings and choose a given proposed opening. Then you click on the computer icon (bottom left)
In both cases that will initiate a game against the computer, by default the highest level. You can change the level , clicking on the computer icon (bottom left). Now you can chose from the available bots a level that suits you.
That´s it! And on top of that, if you have your "starting" position made in the analysis tool just go to the "share" button and copy the URL and save it. So you can always take it again and don´t have to make the situation again.