13 months old

Following

whatever you’re comfortable with. My son has his milk in his MAM bottle and he drinks water from a cup

The NHS website ideally wants you to move to an open cup after a bottle - https://www.nhs.uk/conditions/baby/weaning-and-feeding/drinks-and-cups-for-babies-and-young-children/

However, whilst this is ok for little sips at this age, I'm not sure most just turned 1 year olds would be able to drink a larger quantity of milk easily or efficiently from a open cup. And it's not practical if out. So there needs to be a transition. I've started using a weighted straw cup for both milk and water, as I like that it gives her the independence without being too messy/wasteful. Also she didn't like using other cups I tried. We'll gradually reintroduce an open cup, but I suspect it'll be quite a while until she's using that regularly. My 3.5 year old does use an open cup at home now; maybe since age 2.5ish? However it all depends on the child and how it fits into your lifestyle.
At our nursery, I asked and they said most babies of similar age either use a baby bottle still, a child bottle with teat, or a straw cup.

Can you share a link to the weighted cup? When we use an open cup I need to hold it or it goes everywhere. With the sippy cup we aren’t having much success with milk, only water.

We use the Tum tum straw cups

I use a munchkin weighted straw cup. It's not necessarily recommended by NHS as it has a valve, but I like the less mess, and worked fine for my eldest too. Tommee tippee do ones too, but I found them harder to clean as there's more parts.
Munchkin ones - https://amzn.eu/d/9hr4rr4
Tommee tippee -
https://amzn.eu/d/9mXaXvJ
Or https://www.boots.com/tommee-tippee-superstar-weighted-straw-cup-6m-300ml-10308361
Most supermarkets sell these, but don't always have lots of choice in colours or if you need several.