He drives safely now.
He drives safely nowadays.
For practical purposes, there's no real difference between "now" and "nowadays" in these two sentences. In these two example sentences, any difference at all is hardly worth considering for any practical purpose.
now - primary meaning - at this moment - the present moment
nowadays - also "these days" - refers to that which is current or, in general, what we can call contemporary
You can use "now" in place of "nowadays", but we can't always use "nowadays" in place of "now".
Let's look at some sentences in which there is a stronger contrast between these two words.
Do you want to go shopping now? - We can't say "Do you want to go shopping nowadays?" It's not logical.
Josh never used to like to go shopping, but nowadays he seems to like it, at least a little. - It's possible to use "now" in place of nowadays in this sentence.
Let's listen to that CD now. - It's not possible to use "nowadays" in this sentence because it wouldn't make sense.
Disco music is a thing of the past. It's not popular nowadays. - It's possible to use "now" in place of "nowadays" in this sentence.