Gernerally,when we are using these words as adj:"大多数",it refers to meaning "most/most number of" in English and is used to describe people and something countable,as "大多数人";"大部分" refers to meaning "a large part of" in English and is used to describe uncountable stuffs or datas,as "大部分时间","大部分东西". When you use these two word as an Attributive instead adj like "我们大多数人","我们大多数时间","我们大部分人","我们大部分时间",they do the same thing no matter countable or not. You may imaging the meanings of them as a group of soldiers (like "11111111122",all the numbers are soliders)and a big round apple pie(any favour you like).When we talk 大多数,a chinese native speaker may thinks about 1 in "11111111122" to understand the noun link to 大多数 first;when you talk about time,people think about something like the part of a pie after you take one piece :D to understand the noun link to 大部分.I wish you could get my idea. Besides,there's https://www.italki.com/question/341908 which talked this before.大多数 has only one more 大(which put more stress on" than 多数，so you can read it as same as your question.

let's say.. you can take it just as "lots of"/"plenty of"/"many" ect. so as what you know, It's similar with the difference among those phrases.

“大多数” is generally uesd to modify countable noun.“大部分”can be used to modify both countable and uncountable noun. Hope you can understand me.

meanings of them are similar, but I don't know how to say the different of them, for example we usually say 大部分时间里， but we say 大多数时间里 the chance is lower.

I think don't have different