For things like almond milk and oat milk, it's basically just chopping up the nuts or oats with water, maybe soak them a little and then straining out the pulp, so you get a nice, milky liquid. Some producers add things like protein, vitamin D or B12, to increase the nutritional value and match it with cows milk. Some add sugar, but ofcourse sugarless is beter, especially when using it as an ingredient, so you don't even notice the difference.
I make my own oat milk sometimes and simply blend oats and water together and then strain it. it's really simple and cheap, although you do need a blender for it.
Meat is very bad for the environment and the amount we eat in rich, western countries is waaaaaay too high and very unhealthy. If you skip eating animal products alltogether, you have to make sure to get your protein from things like beans, nuts and tofu (although there's a bit of protein in most plant-based products). You should also be sure to eat leafy greens for the iron and take a B12 supplement. If not available, animal products a few times a week is good enough for the nutrition values.