This is being extremely frugal, not for vegetarians, and something I have not needed to do recently, but as a single parent in the early 80s, when times were hard, I did it all of the time. I would save any residue fat from sausages, burgers, mince etc, put it in the fridge, and cook with it another time instead of buying lard or oil. It was something I learned from my Grandma as I watched her as a child. She also used to do it with bacon but now adays there seems to be a lot of water in it and so it doesn’t keep as well in the UK.
A lot of articles came out later about how bad animal fat was for you, but these have been discredited now, and it is ok in moderation, and actually vegetable oil is also not that healthy either.
Another thing that I used to do was to buy tuna with sunflower oil in rather than brine as I could also use that oil to fry fish cakes in that I made with the tuna.
For some families every penny counts at the moment and so every little trick helps. Some may turn their nose up at doing things like this but for me it is common sense and a way to stretch your budget whilst saving food waste and helping the planet.