Affinity Designer's Grid And Axis Manager is a central tool. You can use them to align paths such as designs created by the app such as stars, rectangles, cogs etc, for the size of paths and much more. How to show it ? view menu and show and to hide, just repeat. The layout can take many forms, there are lots and lots of different settings and probably more hours in the day to go through every possible combination. This is per document, so go to another document and you will find the thing is likely to be OFF. Note, everything in this document can also be applied to Photo
You can always use the default standard or automatic as displayed in the panel settings. If you want to tweak things, go to the panel found in the view menu.
Not a fan of automatic, not even certain how it decides on the setting. Set automatic to OFF and then use basic tab and set the spacing and set the number of divisions between the main lines and set the color of the lines and set the color of the subdivision lines. Set the opacity for both (as required) and finally use it.
You can always just use the basic settings but there are a vast range of other types of other layout which can be used with technical drawing etc. Standard still only gives you by default access to a single line, to get full control you need to deselect the uniform button and then you will be able to modify the second as well (it is not called horizontal or vertical as you can also in some of the features change the angle as well such as in the 'two axis custom'. You can set the spacing for the 1st and 2nd for the main lines but you can also have different divisions for the 1st and 2nd. You can also add in some guttering between the lines to create additional structures.
Click the advanced tab and set the type to something other than standard such as isometric, trimetric, triangular, two etc. The two axis custom gives a decent level of control over all the lines with the gutter, angle etc. As you change the type say to isometric you will notice that some additional functionality appears but overall, all it does is change the angle etc but doesn't allow the angle to be updated. Going to the custom one and you can override things. All kinds of amazing layouts can be created and then used with snapping and objects
You can enter a value for the gutter in the manager, you need to be in advanced panel. You can set the gutter to 20px etc for the first line and second line (basically horizontal and vertical)
There is an interactive cube tab as well, you can move the lines around using the cube. rotate etc. You have all the additional settings at the bottom of the panel for constraining and snapping.
You can go to the top of the panel and right click and select the create preset option and that will save all your custom settings so if you have a gutter / angle etc combination that is super useful to you, you can use the presets to save it for future use. You can also manage the presets and you will see a long list of options such as graph paper, parallel perspective, icons, isometric etc
Go to the snapping panel along the control bar and enable snapping and set snap to grid ON. Go to the pen tool in the toolbar / box and click on layout to add points (you will see a little indicator for the snapping)
After going to all the effort to create a layout. Set the snapping manager ON in the view menu, Use the layout that you have created in the manager. Draw a line with the pen tool say to 8 line's worth. Duplicate that line 7 times and position them on the next line below. Select all the lines and right click and group and then right click and duplicate. Rotate one of the groups via the layer menu and transform and rotate 90. Select all the lines and then go to the pen tool and set the stroke weight (10 pt etc) and then set the cap to square cap and join to miter join and align stroke to center.
All the lines in the artwork can then be grouped into a single group. You can also use the design as a fill by using the expand stroke command in the layer menu but I would suggest you do this to the line right at the start when it is a single line and the opt / alt those lines to create the design (better than expanding later). You can also then use the layer menu and geometry and add and merge to merge all the shapes into a single design.