https://forum.ripper.store/post/1693002

Both require Blendshare, a unity addon, really simple, directions are included in both files!
https://tr1turbo.github.io/BlendShare/

If you really don't want to use blender, there are tools to do what you want to merge clothes with his bulkier body. They're mesh tools or mesh deformation, that work exactly like blender, but a bit simplier and work right in unity! Some are non destructive if you mess up, you can easily revert:
Mesh Lattice 1
https://forum.ripper.store/post/1633492
Mesh Lattice 2
https://forum.ripper.store/post/1663430
Eden Mesh Tools
https://forum.ripper.store/post/1843059
Starmesh
https://forum.ripper.store/post/557126

If this all seems too much, some people make minase appear bulkier, when he just has good normals, light/shading, and textures like these for instance:
Muscular texture
https://forum.ripper.store/post/955939
Muscular texture
https://forum.ripper.store/post/205923
Veins normal
https://booth.pm/en/items/8242177
Normal map
https://forum.ripper.store/post/1690201

Some people people head swap bodies so they'll use Minase head on Hanka or someone else more bulkier, I've even seen people use western eboy bases. Either way, most people use textures, shaders/lighting, normals, and blendshapes for male avatars. You can do it all in unity if you don't want anything too drastic. Just know most people who make male booth assets make them for their default body, so if you edit too much you'll need to do some manual tweaking with mesh tools or conversion tools. Unfortunately, Minase has a lot of assets that don't have his muscular blendshapes, especially shapekeys from blendshare. Anyways, hope this helps! I'm currently doing a Minase for a friend and he wants his to look toned too!