The limit orders do not show on a sim account so you could place a 200 lot order and it will make no difference to what is showing for the actual resting orders on the bid or offer. So if there is fifty contracts actually bid or offered at a price and you place an 200 lot sim order at that price, the DOM or heatmap chart will still show 50 lots at that price.

I believe sim fills are dependent on the platform, some platforms had/have more realistic fills than others. Some platforms had pessimistic fills that required current price to go through the resting orders at a level and trade the next price before filling you so even if last in the queue you would be filled.
Some were optimistic and filled you on first touch but then people could scalp in sim and think things were easy, open an account and lose it quckly unable to get filled.
I understand Bookmap for instance has pretty realistic sim fills as it processes every single order going through, data feeds or platforms that chunk or consolidate data will behave differently
This can be important especially for sim trading thicker products with deeper order books like the ZN, 10 year notes, less important on thinner products like the NQ where price moves around a lot so you would most likely get filled with real orders in the same way as sim.

Some platforms can also give slippage free entries in sim which are unrealistic. Sim trading with stop orders in the NQ to bracket for a breakout to get in with no slippage might work as a sim strategy but less likely to with actual orders as real stop orders to enter would give slippage and real stop-limit orders to avoid slippage would often be unfilled in a thin market if price breaks rapidly with little back ticking.
Topstep for instance had a problem with sim traders exploiting the sim environment to reach the profit targets which is why they brought in a rule where they could reset an account they considered to be "sim abuse".

@owcow

I have only tried Topstep and E2T. The trouble with funding companies is that the profit targets are quite large if trying to scalp. Possibly better opening a small accountof your own and sim trading then trying one lot trading with a broker set daily loss limit. Target not to make much money but try to be consistent and comfortable with out blowing up. Build confidence then increase size later and reduce round turn costs as volume increases.

Funding companies can encourage a swing for the fences trading style scaling in and out of positions to make as much as possible as fast as possible.
If trading equity index futures for example day trading margins can be low enough to only need to open a small account something to bear in mind as a non-US resident requiring funds converted in to US Dollars, or trading in US Dollar denominated products, also not necessarily wanting to send larger amounts to a foreign country.

For funding companies be aware that some have maximum volume amounts that can be traded each day in the sim accounts (to stop HFT sim abuse). Topstep for instance used to be 100 round turns per day maximum which shouldn't be a problem if scalping one lots, but other companies could have lower numbers.

Since funded accounts are demo, how do the order fills work such as limit orders? Do they get filled instantly? Let's say I bid 200 lots limit order, does it show as 200 bids sitting or just 1 bid interested? Or is it an instant fill when traded? Thanks

As an example of a funded program: Apex Funding