If I use BuyDelay=1000 (ms) in the strategy settings on the Buy conditions tab, at what point does the strategy check the balance, at the moment of detection or after the delay expires?

: Strategy settings

Strategies with a non-zero BuyDelay parameter check the balance after the BuyDelay time expires, i.e. just before placing a Buy order. In your case, the check will be done after the 1000 ms delay expires and if the balance is sufficient, the strategy will immediately place a Buy order.