So:

Scrape/clean out all the old silicone / mold

Clear area with 90% isopropyl to kill mold/moisture. Let that dry/breath for 24h

As other have said, buy some bathroom/tile backing rod and jam it in there this will give an anchor point for the next step, the gap here looks about the widest end of a tile spacer so someone likely put a bunch at the bottom for some reason, and leveled the tiles from there, maybe an uneven tub or floor idk.

Once dry, fill your bathtub with water to weigh it down

Fill the remaining _\ gap with Microban Silicone. Microban or similar generic bathroom specific silicone not regular silicone.

This will prevent mold buildup, regular silicone will be black within 3y

Let silicone dry 72H yes it is a pain in the ass to not use the shower for 72 Hours. You can get away with 48 but if possible give it a full 3 days to cure. Plastic line your shower and run a fan if you must.

Buy more silicone than you think, it won't go very far and depending on how well you do it you might need to do a little touch up.

For a nice seam there is a tool to round it or a bit of canola oil on the finger and smooth the silicone out manually leaving a good amount as it will shrink and pull inwards

Good luck.