So I download WhatsApp and make an acc and download matrix and make an acc and then link it ?
You dont download matrix. You download a client for the matrix protocol. The client is your choice but will make a huge impact on your experience. I use fluffychat but I also have element as backup.
You make an account on a matrix instance. tchncs.de is the best public one I can suggest.
You then join their rooms and start asking people for help, politely. I havent used their process but i use the exact same bridges and they work.
Can I then use matrix as a kind of WhatsApp client ?
Whatsapp, signal, telegram and matrix in one app, yes
Can I talk to random users or join WhatsApp groups like I can in the official app ?
I dont know if you need to use a whatsapp client to join a group but you can def talk to anyone and in any group from my experience. this is open source stuff which you need to be aware of. if you're not the product and you dont pay them, then you should help any way you can and be very patient.
Also can I just delete the WhatsApp app after connecting and not worry about it ever ?
You cant "not worry about it ever" on anything, ever. Either you pay someone to make your experience perfect (the mechanic that repairs your bike or your car) or you help yourself and ask for help politely.
Whatsapp can lose the connection (happened to me once in like 3 months) and you will need a whatsapp client to reconnect and then you can forget it again.
Also, its not a billion dollar company doing this so it needs testing and you're the tester so consider submitting your experiences in the form of (rational) bug reports or similar.
**TL;DR: FOSS software will never be "perfect". It is a community effort to improve it. You always pay in this world: Money, Privacy or Work. If you dont, someone else must.