Without a bit more context the best 'do' I can give you is 'do restore the windows'. The best 'don't' is 'don't set fire to them'.
General thoughts:
First cut pitch pine is probably better than any timber you can buy today. Repair not replace where you can.
Where there's rot, splicing in a new bit is better than slathering on the woodfiller - but needs more skill to ensure a good fit.
Where there's rot, cut back to good wood. You know what good wood looks like. Repairs onto wood that looks wrong but feels "hard enough" generally fail quickly.
Think LEAD PAINT. Read up on it before stripping anything.
Wood doesn't rot because it gets wet. It rots because it stays wet. Modern high street paints tend to put a plastic film over the wood that cracks, lets water in, doesn't let it back out. I am a fan of linseed paints - others will disagree.
For sashes, paint everything you can see with the window shut. Don't paint anything else (but wax it if you wanna be a pro). Exception is around the lower areas of the inner stiles and staff beads.
If your building is listed ignore all of the above and agree your plans with your conaervation officer.
Thank you so much for replying! I am trying to load the photo , but technical hitch…just world like to know if a multitool drill type would damage . Can I use filler? Etc I will upload the photo as soon as iCloud lets me!
Thank you so much for replying! I am trying to load the photo , but technical hitch Wondering about the cordless rotary tool. It seems like hard work chipping off the old paint arounf the metal part of the frame.
Do use RepairCare or Timbabuild filler system, don't wince at the cost or use 2-pack 'car body filler' (falls out with movement).
Do use decent paint you've never heard of (Jotun Demidekk or Bedec), don't use Dulux.
I also have a huge regard for 'Bonda' wood hardener, pleasant to use and does an excellent job. https://www.rustins.ltd/bonda/our-products/hardeners/bonda-wood-hardener-
Interesting stuff as it is said to be 'moisture hardened', and there's plenty of moisture involved in these sort of repairs. A huge upgrade on the Ronseal style solvent wood hardeners.
International Paints Ranch Paint lasted really well for me, and have been looking for an alternative that can go over it. Showing promise are the Zinsser Allcoat solvent based paints, and I have also been trialling their water based paint which seemed unusually nice to apply, and gets good reviews. There's more testing to do on that one. Both are self-priming and self-undercoating which makes refinishing a lot more straightforward.
This is the worst of the lot. Will need a replacement sill and a carpenter?
The upper part is a metal frame. Can I use a the multitool to edge off the broken bit do you think?
I suspect that there is precious little there than can be saved. The cill is very likely to be no more than just a few strands of wet and rotten wood that falls apart if looked at too hard. That rot will have also infected to timbers of the frame above and it will very likely fall to pieces when any attempt is made to repair the cill or other parts of the frame. A good carpenter could replicate/reproduce the frame and re-use some of the more durable components but the cost of that would be significant. You should perhaps consider simply replacing this window frame, and any others in a similar state, with 'off the shelf' products that can be quickly and easily installed.
By the way. That tool you are considering is completely inappropriate for anything other than a little light cleaning of the metal work. I suspect it would be a waste of your money.