thunderbold port monitor vs mac book pro 16 inch m3 140w

According to the first Apple Support document linked below,

"If your display provides power to your Mac, you don’t have to connect a separate USB-C power adapter … Your Mac charges over only one port at a time, so connecting multiple power adapters to the USB-C ports and MagSafe 3 port (if present) won't charge your Mac faster."

If the LG monitor is the source of power for the MacBook Pro, the 85 watts will not be enough for your MacBook Pro to "fast charge" ("up to 50 percent battery in around 30 minutes"). The most likely scenario is that the monitor will charge your MacBook Pro at a normal rate. It is possible that if you were doing very intensive work on the MacBook Pro, the machine would use more than 85 watts. Then, instead of charging your MacBook Pro, the 85 watts would just slow down the rate at which your battery charge ran out.

Charge your MacBook Air or MacBook Pro - Apple Support

Fast charge your MacBook Air or MacBook Pro - Apple Support

I'm not a battery expert – or an Apple employee. (This is a user-to-user forum.)

If you still have concerns after reading the Apple Support articles, I would suggest that you get in touch with Apple Support and ask your questions of them.