The letter and the background obviously were originally a grayscale image where the letter had one vertical gradient fill and the background had another gradient, maybe the negation of what was on the letter. Then the grayscale image got a sparse half-toning which contained only black and white horizontal lines, like this:
Sorry for different letter and different gradients.
Then the white areas were colored with a complex blurred image or a gradient mesh. The coloring is not at all simple, it somehow follows the lines of the letter. I made it simple, my colored image is only a gradient:
The white areas of the half-toned image get colored if it's layered with the gradient and the upper one has blending mode Multiply. As well one can delete all white and simply drag the black lines above the color pattern.
Making the half-toned letter image as vector is easy if one has some advanced vector half-toning plugin. It can be created in Photoshop in Bitmap image mode by applying pattern half-toning with pattern "Lines". The result is a raster image. It unfortunately has some roughness and tracing it in Illustrator to vector changes the roughness to wonky lines (tested).
Another possibility to make the vector half-toning with horizontal lines is to blend different height black rectangles. Here's 2 opposite vertical gradients:
The blends are expanded and ungrouped. The rectangles are distributed (or at least tried, see NOTE) to have same vertical midpoint distances.
In the next screenshot both line gradients got the same colored background image (= a rectangle with gradient fill only)
In the next screenshot the black line sets were grouped with their background images to keep the together. A letter b was inserted above the other of the groups:
The letter was used as a clipping mask:
The masked pattern was moved on the rightmost image:
It looks simpler than your example which has much richer background image. Some richness more can be got by changing the gradient in one of the shapes. It can be accessed even below the clipping mask in the layers panel. An example:
The new gradient has still the same colors, it has only different axis. Too big change without any artistic idea looks harsh.
NOTE: The vertical placements of the black rectangles are clearly a little irregular. It's much easier to get the distances right if the blended black rectangles were different width straight lines. It's tried in the next image:
The edges of stroke-only lines have no anchor points, only the midiline have a role in the placement. The used background gradients are simpler, too. They have only 2 stops.