Justinian has the basic approach options down, but I think the question you are really asking is "How do I approach Fox's neutral game."So here we go:
https://www.youtube.com/watch?v=pidjsmje4U8 I will refer to this as the "2GGT"
https://www.youtube.com/watch?v=9fj_kBCyDn8 I will refer to this as the "Abasaga"
The reason I have chosen the above two videos is because they demonstrate a lot of depth about Fox. Spacing with fox, really doesn't exist lol. He kind of just moves around an opponent's zoning options and spacing to get in and hit hard. He has quick frame data which allows him to move in, and thus stay in. The real emphasis for Fox is positioning. A Fox with positional awareness, is imo, the difference between a Fox with good movement, and a great Fox. First of all, especially with Fox, the time spent looking at your character in a match should pretty much be non-existant when in neutral game, because you should be looking at how far away you need to be for your opponent's hit boxes to barely miss you, thus allowing you to get in.
Positioning can pretty much be broken down into ranges. Far (basically a stage away), mid (half a stage away), and short (right next to your opponent). Good positioning is quite match up specific, so I'm going to talk about the two matches above briefly to give you an idea of how you should be approaching this when learning it. I will divide positioning for Fox into two general categories: aggressive positioning and passive positioning.
Aggressive positioning is what you see more in the Abasaga video. Larry Lurr positions himself at mid-range to close-range against Diddy nearly at all times. He also preferentially takes mid stage, with Diddy's back toward the ledge. So why space mid-range to close against a Diddy Kong? The reasoning is that Diddy Kong is slower than Fox. This means that you can react to him dashing toward you, faster than Diddy can react to Fox dashing toward him. This opens up Fox's options. When you go for a pivot grab, I highly doubt you will succeed in doing so by dashing across the stage and attempting it. If, instead, you dash to mid-range and space a back air to threaten them and slowly move in on their space, suddenly you will find that a pivot grab is a good possibility to land. If you approach the game with the binary thought that you are either approaching or not, then your gameplay will quickly become linear and translucent to your opponent. By staying rooted at midrange and constantly watching what your opponent is doing, then your viable approaches increase substantially. Now, when I'm talking about staying midrange, you'll notice that larry Lurr seldom maintains a particular position. He uses Fox's fast fall to do tomahawks and other options. Tomahawking with Fox allows you to bait out option and avoid hit boxes. I cannot tell you how many times I've avoided a Zero Suit samus by just jumping over a grab. The point is that Larry Lurr stays close to Diddy and threatens his space midrange, while moving in and out from Diddy's ranges with different options (the ones discussed above). This is aggressive spacing, since he is mostly threatening space.
Passive positioning is when you threaten midrange, but often retreat to far ranges for lasers. Your goal is to threaten the opponent's space to force defensive options and gain stage control, then utilizing that stage control for damage opportunities. For passive spacing to work, you usually need to first utilize aggressice positioning. The reason is because you even being near an opponent after successfully utilizing aggressive positioning will cause them to feel pressured by you even being near them.
A good example of using Fox's jump to move around your opponent is at 1:28 in the Abasaga video. Larry Lurr pressures Zero's shield, then does a short hop in rather than just dash grabbing, why? ZeRo likes 3 options more than any others with Diddy Kong. He likes rolling, spot dodging, and dtilt. At the range of Fox's jabs, dtilt definitely was a viable fast hitbox for ZeRo to utilize. So Larry Lurr did a tomahawk, which would allow him to jump over dtilt should ZeRo choose that option. What it also does is cause him to not commit. If ZeRo were to have rolled, he easily could have reacted by simply not committing to a grab. Also from ZeRo's initial position, a fair oos was less likely to be viable for ZeRo to do due to Larry Lurr's aggressive positioning all game. Look at this video to find those micro-exchanges, as they will make your Fox really good by understanding them.
There is one more question to answer: Why is such aggressive position near mandatory against a Diddy Kong? The reason is because of fricking banana. When Diddy has a banana in his hand, he has an instantaneous option oos that yields great punishes against Fox. The Fox:Diddy Mu when Diddy has a banana is without a doubt in favor of Diddy, so you want to stay close to Diddy so that you can always react to a banana toss. On the other hand, you do need to mix up into passive positioning occasionally so that you can bait your opponent into overcommitting while keeping your own neutral game unpredictable.
So, if you really take the time to understand "how" to position with fox, then his neutral game becomes astoundingly powerful. Now dash attacks will land, pivot grabs are viable, cross up nair is more viable, and bair to threaten space makes more sense to do.
Positioning with Fox requires you to really understand your reaction times well in order for you to utilize it to its fullest, so good luck. I hope this helped