Hello OP, I'm very sorry to read about your lovely mum. You must feel so shocked.
I'm afraid I have no personal experience of either the logistics or etiquette surrounding Zoom calls but notwithstanding some of the practical considerations others have already mentioned I do understand your hurt and disappointment at the way a good proportion of your friendship group has acted since finding out about your bereavement.
Certainly, I would have expected, or at least hoped, that 'a few weeks' would have been long enough for my closest friends to contact me in one way or another to both acknowledge my loss, and/or to show some concern for how I was bearing up (or not). I can easily imagine feeling rather upset and bewildered in your situation by their silence and if they were people I felt close to, perhaps making excuses for them (not least Covid related worries for example) rather than consider they simply didn't care or that our friendship was less important to them.
With that in mind I can also imagine joining that Zoom call thinking surely if I'm right there in front of them, those who've not yet been in touch will finally say something. Surely they'll at least tell me it's lovely to see me. Surely they'll simply ask me how I am even if they feel too awkward or whatever to refer directly to my mum's death. I might even have thought that they'd have contacted me privately after I left to ask if I was alright.
So yes..... I totally get how angry you feel. Because you placed an opportunity for your friends to be..... well, friendly and compassionate, right in front of them.... and they still didn't make that connection with you even though you'd made it 'easy' for anyone who may thus far have been avoiding you because they 'didn't know what to say'.
It's crap, it really is. Of course bereavement and grief is an emotive situation and obviously, most people wouldn't want to make someone going through the process feel any worse than they already do. But why oh why does anyone imagine that completely ignoring someone is the best course of action to take? A simple sympathy card is better than nothing and takes very little effort if you really don't have it in you to pen some personal words. On the other hand if you really are genuinely scared of saying the 'wrong thing' then what on earth is wrong with literally saying 'I don't know what to say but my heart's breaking for you and I want you to know you're in my thoughts'. The way I look at it is that regardless of any awawardness I might feel, the bereaved party will be feeling a million times worse so if I care about them I can bloody well make a bit of effort, put my discomfort to the side and reach out in some way. It doesn't have to be major, or involve grand gestures, particularly if you have pressing issues happening in your own life but it's about acknowledging that someone you allegedly care about has received this body blow. I think it's natural when you've been bereaved to feel very out of kilter, and see the world carrying on as before, as if everything was 'normal' when for you, personally, it's anything but. That can be very hard to come to terms with as it seems so surreal. When others fail to mention the person who's died and/or show concern for those left bereft it can almost feel like they're unspokenly suggesting they never existed at all.
I really don't know what else to say to you OP at this point. I'm sorry you've found out you have fairweather friends at a time you genuinely need them. I personally don't think there's any (or at least not many) excuse(s) for their attitude. And presumably if they were going through major traumas of their own right now which justified their silence you'd almost certainly know about it. And the chances of all four of them being in that boat are particularly unlikely. FWIW I think they've been incredibly thoughtless, unkind even. And now you're also going to be dealing with the loss of friendships because even if you do remain in touch I suspect you won't easily be able to get over this and the dynamic will alter anyway.
All I can suggest is that if you need to, you reach out and depend upon those friends and family who have contacted you rather than waste any further emotional energy fretting about those who haven't. Possibly easier said than done I admit. This is such an awful time to be trying to learn to live with a bereavement because so many of the usual interpersonal rituals (like well attended funerals and wakes) behaviours (hugs, meeting indoors) and activities which might otherwise provide a little respite from overwhelming thoughts are impossible. Everyone knows that. And it's perhaps even more important than usual therefore that you reach out when someone you know is grieving. I really do think you've been badly let down and your anger is understandable. All this upset, hurt, re-evaluation of friendships could have been avoided by what, taking half an hour or even less to show a little concern. It's very sad.