Possibly the simplest and easiest way for you to achieve this is to use the Messages App that is built into MacOS. You both need an Apple ID and a good/robust broadband connection with as much upstream/downstream bandwidth your ISP can provide on the package you have.
How does it work? You send your mum a message on the Messages App. She receives it and from there she selects one of the options on the Buddies pull down menu (as below):

Basically she's asking you to access/use/control her screen. The other option is for you to ask her to share her screen. Either way you or she will get a message asking to accept or decline the invitation. Once you accept you can control her screen as well as talk to her at the same time.
In reality (which is why I mentioned the broadband bandwidth requirements) it can be clunky and echoey. But it can get the job done well enough provided you're patient with it. Other alternatives are Teamviewer and similar. If you're familiar with Zoom or Teams I think there's a way to use them as well although it's not something I've ever explored fully. Whichever method you end up with I'm sure it will give you and your mum what you want.
What I would recommend you not to use is ARD (Apple Remote Desktop) the subject of this forum. You'd have to pay for it and you would have to have some knowledge of firewalls, IP addressing and general networking expertise. No offence intended but I doubt this is something you would want to tackle?
Good luck.