mat, matting, (kokuji)
蓙 centers on a woven floor covering: a mat or matting. It is a kokuji (Japan-made kanji) used specifically for the word ござ (goza), a type of straw mat.
蓙 is a kokuji formed by combining 艹 (grass) with 座 (seat), suggesting a grass seat or mat. The exact historical development is uncertain, but the component choice fits its meaning.
The grass radical 艹 sits on top of 座 (seat), like a woven grass mat placed over a seat. Picture a straw mat covering a wooden bench.
For ザ, imagine sitting on a mat and saying 'ahh' in comfort: 'za' is the sound of relaxation, like the 'za' in 'pizza' but without the 'pit'. The mat makes you go 'za'.